When SIP ALG is turned on in many cases you will see the SIP phone rings but you cannot answer the call. Another side effect is the SIP Phone will show registered with the Public IP address of the router instead of its Private NAT IP address. When the SIP ALG is turned off, the normal Private address should appear after reregistration and work properly.
If SIP ALG is turned off and you still experience the problem, there could be another setting in the router effecting NAT or the change in setting is not staying permanent due to a save issue or router defect. Talking Platforms has a sophisicated mechanism for solving NAT problems and does not require the help of a router to address SIP issues.