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Custom phone numbers

Custom Phone Numbers

Bring your own numbers (BYON) to Synthflow and connect your carrier, trunk, or PBX directly to our network. This guide explains the options, requirements, and steps for adding custom numbers.

What is a Custom Number?

A Custom Number lets Enterprise and Agency customers connect their own phone numbers through SIP trunks or PBX systems.

With this setup, Synthflow AI Agents act as an extension of your trunk/PBX—giving you more control over call routing and number management.

👉 Learn more about how this works on our SIP Integration page.

Supported Integrations

Synthflow integrates with a variety of carriers and vendors such as RingCentral, Telnyx, and Twilio.

  • See our integration guides for setup help.
  • We’re constantly adding new partners—ask your account manager if you don’t see your vendor listed.

Adding Custom Numbers

When adding a custom number in the Synthflow portal, you’ll be asked to configure several fields:

  • Phone Number – The CLI your AI Agent will use. Must be in E.164 format (+<countrycode><number>).

  • SIP Domain – The destination domain for outbound calls.

  • Username – Authentication username (if required by your carrier/trunk/PBX).

  • Password – Authentication password (if required).

  • Outbound Proxy – Use when:

    • sending requests via a mid-registrar/proxy
    • using a non-standard SIP port (e.g., 5065) Format: fqdn/ip:port.

  • Auth Username – Used for REGISTER requests from Synthflow to your carrier/trunk/PBX (optional, depending on carrier).

  • Origination URI – The Synthflow SIP address and port we use to send traffic.

FAQs

How do I enable this?

  • Go to your workspace.
  • Click Phone Numbers in the top-right.
  • Select New Phone Number → Import a Custom Number.

If you see an Upgrade button instead, this feature requires an Enterprise account. Please contact your account manager for details.

I just upgraded to Enterprise—when can I add numbers? Immediately. If the option doesn’t appear, try logging out and back in. If it still doesn’t show, contact support.

Do you support non-standard SIP ports? Yes. Use the Outbound Proxy field when adding a number to specify a custom port (e.g., fqdn.example.com:5065).

Need Help?

If you run into issues or have questions about supported carriers, contact your account manager or reach out to our support team.