Troubleshooting Dropped Calls on VoIP Systems

Calls that disconnect unexpectedly ('dropped calls') on VoIP are frustrating and undermine confidence in the phone system. Systematic diagnosis is needed to identify whether the cause is network, PBX, or carrier-related. Common causes: network instability (check for packet loss during calls using PingPlotter), SIP session timeout (the PBX or NAT device is closing idle SIP sessions — enable SIP keepalives/OPTIONS pings in the PBX trunk settings), NAT timeout (the router closes the UDP hole used for RTP after inactivity — reduce the SIP registration interval to keep the NAT mapping alive), or carrier-side issues. To determine if calls drop at a specific duration (e.g., always at exactly 30 minutes), this strongly suggests a SIP session timeout issue — the PBX is not sending re-INVITE or OPTIONS to keep the session alive. Check the call logs in the PBX for the SIP disconnect reason code. Code 408 (Request Timeout) or 503 (Service Unavailable) point to network/carrier issues; 487 (Request Terminated) typically means the caller hung up. Share call logs with the carrier if the issue is on their network.