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.