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