A 3CX AI receptionist on its own SIP trunk
A generic SIP trunk in 3CX carries calls to Cartesia, so the agent answers as your 3CX AI receptionist and transfers callers back to your team.
Front-line agent
Answers calls 3CX sends over the trunk and passes the rest to your team.
Instructions
Answer warehouse hours, pickup rules and dock check-in. Transfer shipment problems to account management, and callers who ask for someone by name to that person's 3CX extension.
- Handoff
- 3CX team number
Connected tools
- Transfer to 3CXTransfer
What a 3CX AI voice agent can do
Open one to watch a call. The grey lines are the tools the agent calls mid-sentence.
An outbound rule sends calls for the agent's number over a 3CX generic SIP trunk straight to Cartesia, with no carrier in between.
The agent answers what customers ask most, such as warehouse hours and dock rules, and ends the call without a transfer.
Callers who need a person go to a 3CX number, such as a ring group, a queue or a user's extension.
3CX settings
These are the 3CX V20 settings for a trunk to Cartesia and the rule that uses it.
- Trunk
- Admin, Voice & Chat, Add Trunk, then the Generic SIP Trunk.
- Host
sip.cartesia.ai. Cartesia accepts SIP overTLSorTCP, not UDP. 3CX doesn't support TLS on IP based trunks, so useTCP.- Authentication
- IP based in 3CX. In Cartesia, allow your 3CX server's public IP as a source address.
- Outbound rule
- Calls to numbers starting with prefix, set to the agent's number. Make outbound calls on, the Cartesia trunk.
- Extension forward
- The user's Call Forwarding settings, forwarding to External number, the agent's number.
Set it up
A SIP trunk you add in 3CX that points at sip.cartesia.ai. Where 3CX offers no generic trunk, forward an extension to the agent's phone number instead. Vendor details read in November 2026.
Set up the Cartesia side
Create a SIP trunk provider in Cartesia, add your 3CX server's public IP as an allowed source address, import the number 3CX will dial for the agent, and assign the agent to it.
Add a generic SIP trunk in 3CX
In 3CX, go to Admin, Voice & Chat and select Add Trunk. Choose the Generic SIP Trunk. Set the host to
sip.cartesia.ai, use IP based authentication, and set the transport toTCPunder Options.Route calls to the trunk
In Outbound Rules, click Add, match the agent's number with Calls to numbers starting with prefix, and pick the Cartesia trunk under Make outbound calls on. Then forward an extension to that number.
Set the way back and test
Add a transfer rule in Cartesia to a 3CX number that reaches the right person or queue. Call the forwarded extension, reach the agent, and ask for a person.
Before you build
Test the trunk yourself
3CX says it won't assist in configuring an unsupported VoIP provider. Place a test call through the trunk before you send live callers to it.
3CX-hosted systems may not have it
3CX staff on its forum say the Generic SIP Trunk isn't offered on 3CX-hosted systems. There, forward an extension to the agent's number over your usual trunk.
Transfers back use SIP REFER
The agent transfers by sending a REFER over the trunk. If 3CX rejects it, Cartesia keeps the call connected, so test a transfer before you send live calls.
Transfers are cold
The person who picks up gets the call, not the conversation. Have the agent tell the caller where it's sending them before it transfers.
Use cases for teams on 3CX
More integrations
Enterprise-grade security
HIPAA compliant
SOC 2 Type 2
GDPR
3CX AI voice agent FAQs
Does 3CX have an AI receptionist?
Yes. 3CX's AI Receptionist greets callers, collects their details, routes them to the right person and screens spam. A Cartesia agent is a separate option, reached over a generic SIP trunk to sip.cartesia.ai or by forwarding an extension to the agent's number. 3CX has no built-in Cartesia connection.
Can I use my own AI agent with 3CX?
Yes. On a self-managed 3CX, add a Generic SIP Trunk pointed at sip.cartesia.ai and route the agent's number to it. 3CX says it won't assist in configuring an unsupported provider, so check the trunk with a test call. A Cartesia agent calls webhook tools you define in your own systems and uses the voice and model you choose.
How do I connect 3CX to an AI agent over SIP?
In 3CX, go to Admin, Voice & Chat, select Add Trunk, then the Generic SIP Trunk. Set the host to sip.cartesia.ai, use IP based authentication and TCP transport, since 3CX doesn't support TLS on IP based trunks. Add an Outbound Rule for the agent's number on that trunk, and forward an extension to the number.
Can I use a 3CX AI agent without a SIP trunk?
Yes. 3CX-hosted systems may not offer the Generic SIP Trunk, so set a user's call forwarding to an external number, the agent's phone number. The call then goes out over your existing trunk, and that leg costs what your carrier charges.
Can the agent transfer callers back to 3CX?
Yes, with a SIP REFER over the trunk to a 3CX number for the right person or queue. If 3CX rejects the REFER, Cartesia keeps the call connected, so test a transfer before you send live calls. Transfers are cold.
Who bills the minutes on a 3CX SIP trunk?
Agents run at $0.03/min, plus LLM usage. Cartesia adds no telephony fee on a SIP trunk number, so your carrier's rates apply. If you forward over your existing trunk instead, that leg is billed by your carrier too.
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