The issue is this:

With some customers we notice from time to time handsets seemingly being registered with the phone system, but actually not having a registration any more with the phone system.

We have done extensive testing to find out under what conditions this can happen, and we think that we have found the situation that causes this problem.

It happens when RTX handsets are normally registered at the phone system, and the internet connection between RTX base station(s) and phone system is temporarily down.

In that case the registration at the phone system times out, and the phone system expects the base station to renew the registration as soon as the internet connection returns.

However the base station does not notice in any way that the connection is down, nor does it notice that the internet connection is resumed. Even worse, while I presume that the base station will try to re-register after the registration times out, it does NOT remove the SIP registration from the extensions table in the base station. When the internet connection returns, then the RTX base station NEVER again tries to register the extension with the phone system.

Which means that the extension is unregistered with the phone system and therefore cannot be called. Calling from the phone still works, as this does not require a registration.

In my opinion the RTX base station should take care of the SIP registrations at the phone system, and should remove the registration if it times out, if it cannot re-register ( becuase there is a temporary internet outage) then it should continue to try to register, so that when the connection returns, the RTX phones are registered an thus reachable again.

By the way, a restart of the base station(s) solves the problem, but I have cutomers with hyndreds of RTX base stations. Not very practical if you over 40 different sites which need to be restarted and checked every day….