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Thread is not a phone system. Voice AI is an AI agent that has its own phone number, and it joins your call flow the way any other destination would — you forward calls to it. Your carrier, your DIDs, your licensing and your dial plan all stay exactly where they are. So connecting Voice AI comes down to one decision, repeated once per call path: at what point in my existing flow should the AI answer? Everything on this page maps the common answers, and the phone system forwarding directory shows where that setting lives in around thirty phone systems.
Forward live calls only after you have tested the agent by calling its number directly and reviewing the transcripts and tickets it produced. See the testing steps in Answer overflow & after-hours calls and Replace your phone menu with the Auto-Attendant.

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Which Thread number goes where

Voice AI gives you two agents, each with its own number, requested under their respective tabs in Voice → Agents:
  • The Attendant answers the call in order to pass it to your team. It runs in one of two modes, Advanced AI or Minimal AI, which decide how much the agent does before the transfer.
  • The Overflow acts like an AI voicemail, when used in conjunction with Attendant the agent takes over when nobody picks up, and takes a message. Overflow can also be used solo as an AI voicemail that understand the issue, creates the ticket, and lets the caller know they will get a call back. The Overflow agent acts like an AI voicemail. Paired with the Attendant it takes over when nobody picks up and takes a message; on its own it understands the issue, creates the ticket, and tells the caller they’ll get a call back.
Which number you forward to, and which mode the Attendant is in, is what decides the caller’s experience. The forward is identical for both Attendant modes — the mode is a setting on the agent, not a different number. See Minimal vs Advanced AI mode.

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The standard setup: Attendant in front, Overflow behind

This is what most partners end up with, and it needs one forward in your phone system. The Attendant answers everything, creates the ticket, transfers to your team, and hands the call to Overflow if nobody picks up — so your PBX never needs a second rule. Three things about that flow are worth knowing before you build it:
  • The ticket exists before your team’s phones ever ring. The Attendant opens the ticket and attempts contact mapping the moment it answers, so a caller who gives up mid-transfer still leaves you a ticket.
  • The mode decides whether the Attendant speaks. In Advanced AI mode it greets the caller, asks what they need and can run AI caller identification if not mapped by phone number. In Minimal AI mode it says nothing at all — it listens in and routes the call to your queue or ring group. See Minimal vs Advanced AI mode.
  • In Minimal AI mode, the only conversation happens at Overflow. If nobody picks up and the call rolls over, that’s the point where Voice AI triages the caller and collects what your technician needs for the callback. Overflow adds it to the ticket the Attendant already opened rather than starting a second one.
1

Give the Attendant somewhere to transfer to

In Voice → Agents → Attendant → Routing, choose Don’t use menu options and fill in Call queue and extension with the country code and phone number your technicians’ queue or ring group answers on. An extension is optional — use w between digits (for example www90) for a half-second pause if the extension is nested behind a prompt.This destination has to be a number that can be dialed from the public phone network. An internal extension on its own won’t work.
2

Turn on the fallback to Overflow

Under Call time out, set Ring for to how long your team normally needs to pick up, plus about 10 seconds, then enable Use Overflow when no one is available.Request the Overflow number first. Nothing stops you saving this toggle without one, but there’s no agent for the call to land on — and in a menu option, AI Transfer to Overflow stays grayed out and labeled (setup missing) until that number exists.Call time out appears in both Minimal and Advanced AI mode, so a Minimal-mode Attendant can fall back to Overflow too.
3

Forward your support number to the Attendant

In your phone system, point your support DID at the Attendant number and forward immediately — the AI is the thing that should answer, so there’s no reason to ring first. The forwarding directory names the setting for your platform.
Two ways to send a call round in circles. Both produce symptoms that look like Thread’s fault and aren’t:
  • Don’t build the rollover twice. If the Attendant already has Use Overflow when no one is available enabled, do not also point your PBX queue’s no-answer rule at a Thread number. The unanswered call returns as a brand-new inbound call that Thread can’t recognize as the same conversation — two tickets, and sometimes two agents on the line.
  • Don’t transfer back to the number you forwarded from. Sending the Attendant’s transfer destination to your main DID bounces the call straight back into the Attendant. Always transfer to the queue, ring group or technician DID.

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Map your existing setup

If your calls don’t flow the way the standard setup assumes, open the pattern that matches how they flow today — each one names the Thread number to use and where the forward goes.
The most common MSP setup, and the easiest place to start: leave business hours alone and replace voicemail only. Point the queue’s no-answer or maximum-wait rule at the Overflow number, and every abandoned call becomes a ticket with the caller’s issue in it.When that’s proven, add the Attendant in front using the standard setup above.
Replacing the menu is the better end state, and it’s what Advanced AI mode is for. Forward the whole DID to the Attendant and rebuild your options in Voice → Agents → Attendant → Routing → Use menu options. Each option takes a digit, a label, a handling choice — AI Transfer (Record & Transcribe), AI Transfer to Overflow or Transfer only (no AI) — and a destination number.The reason to prefer it is the caller’s experience: one agent greets them, runs the menu, gathers the issue and either transfers them or takes a message — the same voice the whole way through. Keep your old menu in front and callers meet a recording first and an AI second, handed between two systems that know nothing about each other.Keeping your menu is still a reasonable pilot. Forward only the support option to the Attendant number and leave billing, sales and the rest untouched.
Menu options require Advanced AI mode — switching an agent to Minimal turns them off. And if a Voice AI transfer lands in another IVR, turn Technician briefing off under Call handoff: it waits for a human to say their name, which a menu can’t do, and the call breaks.
Overflow works on its own — no Attendant, no second number, one forward. Three shapes are common, and two of them are the patterns above: replacing voicemail on your queue’s no-answer rule, and covering after hours on your closed rule.The third is the one nothing else on this page covers. Run Overflow as the front desk: forward your support number straight to it so nobody answers live. Every caller describes their issue, every call becomes a ticket, and technicians call back from the queue instead of being interrupted mid-task. Partners running a callback-first desk do this deliberately.Overflow can tell how the call reached it, so Overflow Behavior offers separate greetings for a transfer from an IVR and one due to Call Time Out, alongside the default a directly-forwarded caller hears.
Keep the receptionist on the main number and forward only the support DID to the Attendant. To keep one number instead, set the receptionist’s no-answer destination to the Overflow number so those calls become tickets rather than voicemail.
The Attendant transfers to one destination, so point Call queue and extension at a single technician’s DID to get started. A ring group or hunt group is the better answer though — it lets Ring for and the Overflow fallback work across the whole team, and most systems can create one in minutes.
Forward your after-hours or closed answering rule to the Overflow number and leave business hours untouched. Voice AI has no setting that answers during business hours without running the AI — business-hours routing belongs in your phone system.Voice AI does know when you’re closed: Voice → After hours follows the business hours set in Messenger and plays a separate closed greeting, so a number forwarded to Overflow around the clock can greet out-of-hours callers differently. However, a better set up is often making two different agents and using your phone system to switch between them before and after hours. 
You don’t need a PBX at all. Publish the Attendant number as your support number — there’s nothing to forward.Use the Attendant number for this, not the Overflow one: Overflow’s number is a routing destination, and Voice AI says as much in its own settings. For an agent that only takes messages, publish the Attendant number and point it at Overflow rather than handing out the Overflow number itself.For richer call metadata delivered to a PBX, see SIP trunks and caller ID.

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Make the forward in your phone system

Everything above decides which Thread number to forward to. This is the part you do in your own admin console — and it’s the same job on every platform, which is why one set of click-by-click instructions can’t cover them all.

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Find the rule

You’re hunting for two things, on your call queue, ring group or hunt group: Some systems keep both on the group itself, others on a separate answering-rules or call-handling tab. An after-hours forward works the same way: find the closed or after-hours rule and point it at the Overflow number. The phone system forwarding directory names that setting and links the vendor’s own documentation for around thirty platforms, and covers what to do when your phone system isn’t listed by name.

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Check your system allows it

Forwarding to Thread is an external forward, which is the one thing many phone systems restrict by default. If the destination options are all internal — extensions and voicemail only — that’s your blocker, and it’s a question for your phone system provider rather than for Thread.
  • External forwarding is permitted on the object you’re editing — the queue, ring group, auto attendant or resource account. Depending on the platform this is a license, a permission, a class of service, or a global toggle, and it’s the single most common reason a forward silently fails.
  • The number is in the format your system expects — usually E.164 (+15551234567) or your carrier’s dialing format, digits only. Dashes and spaces break some systems.
  • Your transfer destination is reachable from the public phone network, because Voice AI dials it as a normal call.
  • Caller ID is passed through. If your system replaces the caller’s number with its own, Voice AI can’t match the caller to a PSA contact automatically. See Voice AI contact mapping for what to do about it.

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Watch out for voicemail answering the call

No phone system can tell a person picking up from a voicemail box picking up — to the carrier both are an answered call. That’s why so many offer a press-a-key-to-accept option on external forwards: Confirm Calls in FreePBX, Confirm call pickup in GoTo Connect, Required key press on answer in Ooma Office. Voice AI hits the same problem in reverse when it transfers to your team. If a technician’s voicemail answers, the ring timer stops applying and the caller is stuck with it instead of falling through to Overflow. Turn on Technician briefing under Call handoff: a voicemail can’t say its name, so the handoff never completes and the fallback runs. See Technician Handoff — and leave it off if the transfer lands in another IVR.

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Verify before you go live

1

Call your real support number

Dial the number your clients use, from an outside line. The AI should answer within a couple of rings, or the call should reach your team as it always did if you only changed the no-answer path.
2

Let the transfer happen

Confirm the call reaches the right queue and that a technician’s phone rings.
3

Let it time out on purpose

Don’t answer. After Ring for expires, Overflow should pick up, take the message and end the call.
4

Check what landed in Thread and your PSA

You should have a ticket on the board and source you configured, with a transcript, a recording and an AI call recap. If tickets aren’t appearing, see Voice AI troubleshooting.
5

Test out of hours

If you configured After hours, repeat the call outside your Messenger business hours and confirm the closed greeting plays.

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Next

Phone system forwarding directory

The setting to look for and the vendor’s own documentation, for around thirty phone systems.

Voice AI troubleshooting

What to check when calls don’t arrive, transfers fail, or tickets don’t appear.
Still stuck? Contact Thread support at help.getthread.com with the name of your phone system and which rule you’re trying to change.