Troubleshooting
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      1. ThinkTel Help Centre
      2. SIP Trunking
      3. Troubleshooting

      Most common call dropping issues.

      This article will explain the most common call dropping SIP issues and solutions for each scenario.

      Calls are dropping but only sometimes.

      This may indicate that the phone is not reregistering on its set interval with the ThinkTel switch (3600s). The switch will assume that the device is no longer present and will drop any calls that may be in progress.

      • Verify the latest firmware is being run on the phone/adapter.

      • If you are running your phone behind a NAT firewall that is SIP aware, packet mangling may be occurring.

      • If you have other IP handsets or ATAs, check if they are also configured with the same phone number. This could create registration conflicts and sudden call drops while the other devices register on the same number. 

      Calls are dropping after a few seconds of being answered.

      This typically means there is no reply to the 200 OK from the Thinktel switch. The 200 OK indicates the call has been answered.

      The phone could possibly be:

      • Responding to the 200 OK but is routing to the wrong place.
      • The phone is responding on the wrong port. ie: port 0
      • The phone is responding back to the wrong IP address.
      • A SIP aware firewall may be mangling packets or not forwarding SIP ACKs in NAT scenarios.

        We have seen this behavior in Juniper firewalls.

      Calls drop 15 minutes into call

      If you call drops after approximately 15 minutes, you may not be responding to the SIP re-INVITE. Our switch performs a SIP re-INVITE every 15 minutes for any active calls in your SIP Trunk. It's a way of checking if the call is indeed still active. If an appropriate 200 OK is not received back, we disconnect the call.

      Calls drop 30 seconds into call

      This may occur if you are using an ATA or IP Phone peered directly to Thinktel's switch.

      What happens is our switch sends a SIP 'keep alive' every 30 seconds to make sure the call is still active. It does this by sending a SIP re-invite for the live call, in which we expect a 200 OK response, that we then ACK back. If no response is received, the call is terminated by the switch.

      This could be caused by:
      - A SIP aware firewall that is mangling packets.
      - NAT issues on your router and does not support SIP properly

      Calls drop 50 minutes into a call.

      This may occur if we don't get a response to our 'keep alive'


      What happens is our switch sends a SIP 'keep alive' after 50 minutes to make sure the call is still active. It does this be sending a SIP reINVITE for the live call, in which we expect a 200 OK response, that we then ACK back. If no response is receive, the call is terminated by our switch.

      We would have to investigate the call logs to confirm the above and if confirmed then the settings on your PBX as to why it is not responding to reINVITEs.

      Calls drop after 1-2 hours on every call

      Problem: Calls drop at the same time, every time. i.e after 1 hour or most often 2 hours.

      Solution: Most PBX's have a max call duration (default setting 120 minutes) to help prevent toll fraud. You will have to locate this setting and increase the call duration. Or be aware of this setting and place your call again.



      We hope this info was a help. If you have any problems or questions, please let us know.
      You can open a support ticket or contact us atphone-icon-blue 1-888-852-2381 for additional assistance.