To solve this, it would be best to deactivate the SIP ALG protocol in those routers or firewalls where possible. Incoming calls that do not ring, or do not reach the terminal.Problems caused by breaking the SIP Protocol Paradoxically, this “SIP ALG” functionality designed to improve the NAT function in SIP communication, what it does in many cases is to break the SIP protocol. The most common problem is that some routers or carrier firewalls implement a feature called “SIP ALG”. There are different reasons why the confirmation response issued by the SIP terminal to the PBX does not reach back the PBX. If the PBX or Call Server does not receive this confirmation, it may end the call after some time. This way, the Call Server unit knows that communication is in progress. The reason this happens is that according to the SIP protocol the phone must respond with an ACK to indicate that it has received and answered the incoming call properly. This can happen when we use softphones or IP terminals with SIP session initiation protocol. Calling parties are able to communicate each other. But all is suddenly cut off while talking for a few seconds.
Normally, you receive and set up a call phone correctly.
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