I believe I have identified the issue after receiving some advice from a user on a different forum. The registration interval on my telephones is set to a value that is likely too long. I notice that if I place a call from one of my IP telephones to my mobile telephone then it is able to receive incoming calls again until the registration expires. The other IP telephone will not receive calls until I place a call from it too and stops receiving incoming calls once the registration interval expires. I didn't experience this issue in the past which is what puzzles me but this seems to be the issue regardless. I am using the default value of 3600s. What should that value be if I want to be able to receive calls at any time? Is it possible to keep the phones registered without them going through a registration cycle?
That sounds logical, and if the case, I would think your VOIP provider could see that the phones are unregistering in their logs. That could explain why no SIP INVITE would be sent to the phone. Your provider should be able to help identify how to keep your phones registered.
My provider said I could lower the registration interval to 300 and that if I still have problems I can do port forwarding. The port forwarding method has worked flawlessly so far.
Hodor, I am surprised that even at 180 seconds the issue still persists for you. I would update the software back to the latest version and set the interval lower. If that doesn't work, then you will need to assign static, internal IP addresses to each of the telephones. Then you can proceed with creating a port fowarding rule for the telephones. If port forwarding doesn't work for you, then the issue lies elsewhere. I am always concerned with security when it comes to port forwarding but there should be no major risk presented by port forwarding for an IP telephone. I would make sure strict user validation is on also depending upon your setup.