How to integrate voice agent with my client's business phone number

Support Help
1

how do I actually connect my voice agent to my client’s business phone number, so that when his business gets a call and he can’t answer, the voice agent takes over the call after a certain number of rings, or instead of going to his voicemail?

2

Hello

Import the number via Elastic SIP trunking — set up SIP trunking with their telephony provider (Twilio, Telnyx, Vonage, etc.), point the number to Retell’s SIP server sip:sip.retellai.com, then import it to Retell and bind your agent.

For more details you can see our (Custom Telephony Overview - Retell AI) Guide.

Thank You

3

Thanks Mark, but that’s not quite specific enough. I have a Twilio number that I purchased for the agent, but my business owner clients will all be using providers like Verizon, T-Mobile, etc. How do I set it up so that when someone calls their business the AI agent will pick up either after a certain amount of rings, or instead of their voicemail?

4

Hey @michaelpd42

You have two common patterns:

Option 1 — AI answers when the client doesn’t pick up (recommended)
Have your client enable Conditional Call Forwarding on their mobile line so unanswered/busy/unreachable calls roll to the Twilio number instead of their voicemail. Standard US carrier codes dialed from the client’s phone:
Verizon: *71<TwilioNumber> then call (no-answer + busy forwarding). To turn off: *73.

  • T-Mobile / AT&T / most GSM:
  • No answer: **61*<TwilioNumber>**<rings># (rings = 5/10/15/20/25/30)
  • Busy: **67*<TwilioNumber>#
  • Unreachable: **62*<TwilioNumber>#
  • All three at once: **004*<TwilioNumber>#
  • Disable all: ##004#

After that, calls to the client’s normal business number ring their phone first; if they don’t answer in the configured number of rings, the call rolls to Twilio and the Retell agent picks up — replacing voicemail.

Option 2 — AI answers every call
Publish the Twilio number as the business line, and either (a) unconditionally forward the carrier line to Twilio *72<TwilioNumber> on Verizon, **21*<TwilioNumber># on GSM), or (b) just have the client hand out the Twilio number directly. This guarantees the agent answers and you can configure a transfer_call tool in the agent to bridge to the owner’s mobile when needed.

A few practical notes:

  • Carrier codes vary slightly — confirm with each client’s carrier before rolling out.
  • Caller ID on the forwarded leg will typically show the original caller’s number (good for the agent).
  • If the client is on a business VoIP / PBX (RingCentral, GoTo, 8x8, etc.) instead of Verizon/T-Mobile direct, the equivalent is “forward on no answer” inside that PBX’s admin panel pointed at the Twilio number — same idea, different UI.
  • Make sure the Twilio number has voicemail disabled on the Twilio side so the call cleanly hits the Retell agent.

Thank You