What causes calls to drop after approximately 30 seconds?

• A. NAT timeout
• B. Codec mismatch
• C. SIP ALG/firewall
• D. Insufficient bandwidth

Answer: (C)

SIP ALG/firewall is the likely culprit because it sits in the signaling/media path and tries to rewrite SIP messages and RTP payloads as traffic traverses NAT. When a call is established, the endpoints exchange SIP/SDP information and set up the media path. If the router’s SIP ALG misinterprets or rewrites those details, the RTP streams can get directed to the wrong address or ports, or the signaling can become inconsistent with the actual media path. The result is that the call appears to connect, but the media can’t flow correctly after a short period, and the session is torn down around the 30-second mark as components time out or detect the mismatch. Disabling the ALG or ensuring direct, properly traversed media paths (for example, with ICE/STUN/TURN) typically resolves these drops.

NAT timeout would usually show up as a broader connectivity issue tied to mapping lifetimes rather than a consistent 30-second drop after successful setup. A codec mismatch or insufficient bandwidth might cause call setup failures or ongoing quality problems, but they don’t typically produce a uniform drop at a fixed interval like this.