LAUNCHTelephony

Phone Numbers

Buy numbers, import your own, and manage them across your agents.
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Phone Numbers overview in Synthflow

Synthflow lets you manage phone numbers for inbound and outbound calls and SMS messaging. You buy numbers directly for quick testing, import your own through a SIP trunk or PBX, or bring numbers from your own Twilio account. Configure active numbers and addresses in the Phone Numbers tab.

Each agent needs a phone number to place or receive calls. From the dashboard you can assign numbers to agents, track call and SMS activity per number, and configure voice and messaging settings.

Phone number options

You have three ways to get a number into Synthflow:

One phone number can serve one inbound agent and multiple outbound agents. No two inbound agents can share the same number. For long-term production use, bring numbers from Twilio or another SIP provider so you keep full control over your setup, regional rules, and configuration.

Buy a number

Modal showing the list of available numbers to purchase

To buy a phone number, navigate to Phone Numbers > New Phone Number > Buy a Number.

Synthflow offers U.S., Canadian, and Australian numbers for quick testing directly through the platform. Phone numbers cost $1.50 USD per month.

Manage your subscription

After purchasing a phone number, manage your subscription to that number under Admin → Workspace Settings → Plan & Billing → Billing Portal.

Australian numbers

If you want to purchase an Australian number, you need to provide an address. Some local authorities require users to provide their name and address to purchase and own a phone number.

To register an address, go to Phone Numbers > Addresses. See Addresses for details.

Custom numbers

Modal showing the import custom number setup

Bring your own numbers (BYON) to Synthflow and connect your carrier, trunk, or PBX directly to our network. A custom number lets Enterprise customers connect their own phone numbers through SIP trunks or PBX systems.

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

Supported integrations

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

  • See the integration guides for setup help.
  • We are constantly adding new partners. Ask your account manager if you don’t see your vendor listed.

Add a custom number

When adding a custom number in the Synthflow portal, you 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, or PBX).
  • Password: Authentication password (if required).
  • Outbound Proxy: Use when sending requests via a mid-registrar or proxy (format: sip:fqdn/ip:port), using a non-standard SIP port such as 5065 (format: sip:fqdn/ip:port), or using a specific transport such as TCP (format: sip:fqdn/ip;transport=tcp). When specified, it overrides SIP Domain as the destination for all outbound calls and registrations.
  • Auth Username: Used for REGISTER requests from Synthflow to your carrier, trunk, or PBX (optional, depending on carrier).
  • Origination URI: The Synthflow SIP address and port we use to send traffic.

To enable custom numbers, go to your workspace, click Phone Numbers in the top-right, and select New Phone Number → Import a Custom Number. If you see an Upgrade button instead, this feature requires an Enterprise account. Contact your account manager for details.

Bring your Twilio numbers

If your numbers already live in Twilio, connect your Twilio account instead of importing each number over SIP. Numbers stay in Twilio while routing, recording, and call transfers run through Synthflow.

To connect your account, open Admin → Workspace Settings → Integrations → Twilio and paste your Twilio Account SID and Auth Token. For the full setup, including regional routing and compliance, see the Twilio integration guide.

Addresses

Addresses are required when purchasing phone numbers in most countries due to regulatory compliance. Provide a valid address before acquiring a number.

To set up an address:

  • Create an address in the dashboard under Phone Numbers > Addresses.
  • Assign the address to any phone number that requires one.
  • Verify compliance to ensure smooth activation.

Once assigned, the address is linked to your active numbers and can be managed as needed.

Address validation may take some time to complete.

FAQ

Purchased numbers cost $1.50 USD per month. Manage the subscription under Admin → Workspace Settings → Plan & Billing → Billing Portal.

You can buy numbers in the U.S., Canada, and Australia directly through Synthflow. For local numbers in other regions, import a custom number from your own carrier or bring your Twilio numbers.

Go to your workspace, click Phone Numbers in the top-right, and select New Phone Number → Import a Custom Number. If you see an Upgrade button instead, this feature requires an Enterprise account. Contact your account manager for details.

Immediately. If the option doesn’t appear, try logging out and back in. If it still doesn’t show, contact support.

Yes. Use the Outbound Proxy field when adding a number to specify a custom port (for example, 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.