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This article describes the reason why SIP calls are disconnected within 30 seconds after being answered
Scope
FortiGate.
Solution
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.
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.
