SIP trunks and numbers

Comcent does not sell phone numbers or minutes. You bring a SIP trunk from a carrier such as Twilio, Telnyx or Bandwidth, and your carrier bills you for calls. In Comcent you then:

1. create a SIP trunk that tells Comcent how to reach the carrier and which carrier IPs to accept calls from, and
2. add each number you bought from the carrier, link it to the trunk, and give it an inbound flow.

Only admins can manage trunks and numbers.

Create a SIP trunk

Open Sip Trunk in the sidebar and click Create.

Field	Description
Name	Any name, 3–25 characters, e.g. twilio.
Provide Outbound Credentials	Tick this only if your carrier authenticates outbound calls with a Username and Password instead of by IP address. When the carrier challenges a call, Comcent answers with these credentials.
SIP Proxy Address	Your carrier’s termination address, e.g. mytrunk.pstn.twilio.com. A domain, IPv4 address (optionally with :port) or IPv6 address, without a sip: prefix. Outbound calls are sent here.
Inbound IPs to whitelist	Comma-separated IPv4 addresses or CIDR ranges your carrier sends calls from. Inbound calls from any other address are rejected with 403.

On the carrier’s side

1. Allow your server’s IP. Add your host’s public IP as <YOUR_IP>/32 to the trunk’s IP allow list (on Twilio: IP Access Control Lists on an Elastic SIP Trunk).
2. Send inbound calls to Comcent. Set the trunk’s origination URI to sip:<YOUR_DOMAIN>?transport=udp.
3. Route your number to the trunk, so calls to it reach Comcent. Every carrier names this differently; on Twilio you assign the number to the Elastic SIP Trunk.

Add a number

Routing a number to the trunk at the carrier is not enough: Comcent also needs to know the number and what to do with calls to it. Open Numbers and click Add.

Field	Description
Name	A friendly label, 3–100 characters.
Number (E.164)	The number in E.164 format, e.g. +15551234567. A number can exist only once on an instance.
SIP Trunk	The trunk that carries this number’s calls. Outbound calls using this number as caller ID leave through this trunk.
Allow Outbound if destination matches regex	Optional regular expression for the destinations that may be dialled from this number, e.g. ^\+1[0-9]{10}$ for US numbers only.

Below the form is the flow editor for the number’s inbound call flow. Click Add to save.

Outbound caller ID

When a member places a call to a phone number, Comcent chooses the caller ID in this order:

1. the number the member picked in the dialer;
2. the member’s Default Outbound Number, set on their profile;
3. the organisation’s default number, set with Set As Default on the Numbers page.

If none of these exists, the call is rejected. The call leaves through the SIP trunk of the chosen number.

Tip

Outbound calls failing with 403 from the carrier almost always mean your server’s public IP is missing from the carrier’s allow list.