Troubleshooting Tip: How to overcome a situation where SIP calls are disconnected within 30 seconds after being answered

SIP calls getting disconnected within 30 seconds after being answered is a symptom that the 3-way handshake for the INVITE/call establishment (INVITE/200 OK(INVITE)/ACK) did not complete and that the remote party did not receive the ACK packet in within 30 seconds after sending the 200 OK message - the remote party is the one disconnecting the calls.

Below is the call flow for SIP:

This issue is not a FortiGate issue:

• For outbound calls, the issue needs to be analyzed on the internal PBX/Phone side.
• For inbound calls, the issue needs to be analyzed on the remote PBX or VoIP provider's side.

Related article:

Technical Tip: VoIP and SIP configuration and troubleshooting resource lists