--- title: Connect a SIP trunk description: "Add a number over your own SIP trunk: pick a carrier, detect the server, set gateway, registration and concurrency, then test that the number answers." --- Voice agents A SIP trunk connects Callab AI to your carrier or PBX over IP. You describe the trunk once, with its gateways and credentials, and attach a phone number to it. Further numbers on the same trunk reuse that configuration. :::note[Before you start] From your carrier you need the gateway host or IP and port, the protocol (UDP, TCP or TLS), and, if the trunk registers, a username, password and realm. On the carrier side, allow traffic from Callab AI's public IP; the form shows it under **Register Public IP in Contact**. ::: ## Add the first number on a new trunk 1. **Choose the type** Under **Phone Numbers**, choose **Add Phone Number**, then **Custom SIP Trunk**. 2. **Pick a carrier** **Connect via SIP trunk** lists 30 carriers and PBXs with a search box. Pick yours, or **Custom / other** to start blank. The pick pre-fills the trunk's **Nickname**; everything else you enter yourself. See [Carriers](/deploy/phone-numbers/carriers). 3. **Number** **Create New SIP Trunk** opens, a wizard in five steps: **Number**, **Trunk & gateways**, **Authentication**, **Test connection** and **Summary**. Type the **Phone Number** as the carrier expects it and a **Nickname**. The nickname is also the trunk's carrier name, so spaces and punctuation are dropped on save: `Docs: SIP` is stored as `DocsSIP`; use hyphens instead. **Concurrency Limit** is the number of simultaneous calls allowed on the trunk, `20` by default; an outbound campaign never dials more lines than this. Set **Send the leading + to the carrier** here too. 4. **Trunk & gateways** **Server type** identifies what is on the other end: enter the host and port you were given and choose **Detect**. A recognised server picks the provider template and fills in the first gateway; otherwise choose **Skip detection and pick a template** and pick the closest match. Then check **Gateways**: each one has an **IP Address / Domain**, a **Port** (`5060`), a **Protocol** and a direction. A trunk needs at least one gateway. Add a second for a backup route. 5. **Authentication** Set the **SIP Configuration Options** in the table below. Tick **Requires Register** when your carrier authenticates by registration; **Registration Settings** appear. 6. **Test connection** The test runs as soon as you reach the step: each gateway gets a SIP OPTIONS ping, plus a registration attempt when registration is on. **Probe again** repeats it. If a gateway fails you can still go on with **Continue anyway**, but calls through it may fail. 7. **Summary and create** **Summary** shows what you entered and the **Connection test** result: **Passed**, **Not run**, or failed and saved anyway. Choose **Create Phone Number**. ![The Connect via SIP trunk carrier picker.](/media/deploy/sip-carrier-picker.webp) ![Create New SIP Trunk at its first step, Number.](/media/deploy/sip-trunk-form.webp) ## Fields ### Trunk | Field | Default | Notes | |---|---|---| | **Phone Number** | | The number in the form your carrier routes. | | **Nickname** | Carrier name | Shown in lists and campaign pickers. | | **Concurrency Limit** | `20` | Maximum simultaneous calls on the trunk. | | **Send the leading + to the carrier** | Off | On: numbers go out as `+15551234567`. Off: `15551234567`. Match what the trunk expects. | | **Requires Register** | Off | The trunk authenticates by SIP REGISTER. Reveals the registration fields. | | **Register Public IP in Contact** | Off | Puts Callab AI's public IP and port in the SIP Contact header. Turn it on when the carrier needs a routable contact; allow that IP on your side. | | **Tech Prefix** | | Jambonz trunks only. | | **Diversion Header** | | Passed through when the trunk enables Diversion. | ### Registration Settings Shown when **Requires Register** is on. | Field | Notes | |---|---| | **Register Username** | The SIP username. | | **Register Password** | The SIP password. | | **SIP Realm** | Optional, for example `sip.example.com`. | | **From User** | Optional override of the From user. | | **From Domain** | Optional override of the From domain. | ### Gateway | Field | Default | Notes | |---|---|---| | **IP Address / Domain** | | `192.168.1.1` or `sip.example.com`. | | **Port** | `5060` | | | **Protocol** | `UDP` | `UDP`, `TCP` or `TLS`. | | **Inbound** | Off | The gateway sends calls to Callab AI. | | **Outbound** | On | Callab AI sends calls to the gateway. | | **Active** | On | Untick to keep a gateway configured but unused. | | **Options Ping** | Off | Send SIP OPTIONS to check the gateway is up. | | **Pad Crypto** | Off | Pads SRTP crypto attributes for gateways that require it. | Each gateway matches a single address: there is no netmask to widen it. For inbound calls, the host has to be the exact address your carrier sends from; if the carrier sends from several, add a gateway for each. ## Add another number to the same trunk Repeat **Add Phone Number**, **Custom SIP Trunk** and a carrier. In **Create New SIP Trunk**, choose **Use an existing trunk instead**. In **Add New Phone Number to SIP Trunk**, type the **Nickname** and **Phone Number**, then under **Select SIP Trunk** pick an existing number whose trunk this one should share. **Add to SIP Trunk** creates it with the same gateways and credentials. ## Verify - The number is in the list with **Provider** `Custom SIP Trunk` and **Status** `active`. - Open its details. **Gateways** lists what you entered; **Provider configuration** shows **Requires register** and **E.164 leading plus** as set. - Create an [inbound campaign](/deploy/campaigns/create-an-inbound-campaign) on the number and call it from a phone. The call appears in **Call Logs** and the **Concurrency** bar on the number's page moves while it is up. ## Troubleshooting - **Calls never connect.** Check the gateway host, port and protocol against the carrier's SIP settings, and that the carrier allows Callab AI's public IP. If the trunk registers, confirm the credentials and realm. - **The carrier rejects the dialled number.** Flip **Send the leading + to the carrier**. Carriers are split on whether they want `+15551234567` or `15551234567`. - **A campaign dials fewer lines than expected.** It is bound by **Concurrency Limit** on the trunk. Raise it only as far as the carrier allows. - **Inbound calls do not arrive.** The gateway needs **Inbound** ticked, its host must be the exact address the carrier sends from, and an inbound campaign must be running on the number. ## Related - [Carriers](/deploy/phone-numbers/carriers) - [Phone numbers overview](/deploy/phone-numbers/overview) - [Create an inbound campaign](/deploy/campaigns/create-an-inbound-campaign) - [Transfer Call](/build/tools/transfer-call)