Why a SIP call drops after about 30 seconds
A call that connects and then drops after roughly 30 seconds often indicates that the successful INVITE response was never acknowledged. One side may already be passing audio while the other continues retransmitting its 200 OK and eventually gives up.
This is different from a session-timer expiry later in a call. Do not change every SIP timer because the duration sounds familiar.
The expected exchange
INVITE →
← 100 Trying
← 180 Ringing
← 200 OK
ACK →
The ACK completes the three-way INVITE transaction. If it is sent to an unreachable Contact or lost through NAT, firewall or routing, the far end may repeat the 200 OK before clearing the dialog.
Read the capture in order
| Observation | Likely direction |
|---|---|
| Repeated 200 OK, no ACK at the same capture point | Find where the ACK was sent or lost |
| PBX sends ACK but carrier does not receive it | Check destination, route, NAT and firewall path |
| Carrier receives ACK yet clears the call | Request its reason and correlate the BYE or error |
| ACK succeeds and the later BYE follows a re-INVITE | Investigate session refresh rather than initial setup |
Inspect the route chosen for ACK
Compare Contact, Record-Route and Route headers with the actual packet destination. With multiple interfaces or NAT, an address that worked for the initial request may not be a valid target for the in-dialog ACK.
Fix the missing message, not the stopwatch. A timer increase can delay failure without restoring a valid signalling path.
Related: SIP call flow, SIP headers and NAT and firewall troubleshooting.
Technical reference
Scheduled for 30 August 2026. “About 30 seconds” is a diagnostic clue, not a universal cause.