ZZetra

SIP trunk and existing numbers

Route calls from a number you already own (PBX, SIP provider or mobile carrier) to your agent.

Overview

You can keep the phone number your customers already call and have your agent answer it, even if the number lives with another carrier, on your own phone system (PBX) or on a GSM gateway. Calls reach Zetra through Twilio, in one of two ways.

What you can do

Keep your existing number and route its calls to your agent instead of buying a new one.

Choose a method

MethodBest for
Call forwardingMobile or landline numbers, and phone systems that can forward calls. Quickest to set up.
SIP domain (SIP trunk)Phone systems (PBX) and SIP providers that can send calls to a SIP address. Keeps the call on SIP end to end.

Before you start

  • The Twilio phone setup done first: a Twilio account connected on your agent's Phone page, with Enabled on.
  • Access to your carrier's or phone system's call-routing settings.
  • A Standard plan or above. See pricing.

Method 1: Call forwarding

  1. Get a Twilio number for the agent

    Follow the Twilio guide so your agent answers a Twilio number. This number stays hidden: customers keep calling your existing one.

  2. Forward your existing number

    In your carrier's or phone system's settings, forward calls to the Twilio number. You can forward all calls, or only unanswered or out-of-hours calls (for example so the agent answers when your team is busy). On most mobiles, dial **21*+14155551234# to forward everything, using your Twilio number.

Method 2: SIP domain

  1. Create a SIP domain in Twilio

    In the Twilio Console, go to Voice → Manage → SIP domains and click Create. Give it a name, for example yourcompany.sip.twilio.com.

  2. Secure it

    Under Voice Authentication, add an IP access control list with your phone system's public IP addresses, a credential list (username and password), or both. Only calls that pass these checks are accepted.

  3. Send the calls to your agent

    Under Call Control Configuration → A call comes in, choose Webhook, paste the Webhook URL from your agent's Phone page and pick HTTP POST. Save.

  4. Point your phone system at the SIP address

    In your PBX or SIP provider, route inbound calls for the number to your SIP domain, using the credentials from step 2 if you set them. The address looks like this:

    SIP address
    sip:[email protected]

    Where to set this depends on your provider. It's usually called the trunk's destination, outbound route or SIP URI.

Check it works

Call your existing number. The agent answers with its initial message, and the call appears in Analytics → Conversations.

How calls work

Calls reach the agent exactly as with a Twilio number: the same greeting, voice settings (Settings → Voice), instructions, data sources and call limits.

Limitations

  • Routing happens at your carrier or phone system. Calls only reach the agent once it's set up to send them.
  • The SIP address must match exactly, and your phone system must pass the IP or credential checks on the SIP domain.
  • Twilio bills SIP and forwarded minutes to your Twilio account.

Troubleshooting

Calls to the SIP domain are rejected

Possible cause: the call doesn't pass the SIP domain's IP list or credentials.

Solution: Add your phone system's public IP to the IP access control list, or check the username and password it sends. Twilio's Monitor → Logs → Calls shows the reason.
Forwarding works but callers hear an error

Possible cause: the Twilio number's webhook isn't set to your agent.

Solution: Follow step 3 of the Twilio guide again to set A call comes in on the Twilio number.

Next steps

Twilio (phone) →Set up and manage the Twilio side.
← Twilio (phone)Slack →