This is a problem I've had with many providers over several years.
It seems when someone complains about a specific service provider, they are often able to fix the problem and release a patched version, but it seems there are several other providers with the same problem and for what ever reason the patch does not fix those ones either. When I bring it up with NCH, they basically say that they haven't really experienced the problem, but if I give them a service provider as an example, they will try and fix the problem. I think NCH is missing the point here in that it seems to be a very widespread issue across many providers.
Don't quote me on this, but I vaguely remember someone saying a few providers send some SIP packets on a different port number (or something to that effect), and this results in Axon sending the re-register requests on the incorrect port so they are not received and eventually the server times-out and calls are no longer taken.