I have a few IAX softphones running on Android. The problem I am having is when the phone moves from one hotspot to another (on WiFi) the registration of the phone drops and does not get reconnected for a long time. In that time incoming calls to the smartphone are dropped (sent to voicemail) instead of causing the phone to ring.
This problem is unrelated to IAX; SIP apps have the same issue. You can reduce recovery time by increasing registration frequency, but that comes at the expense of battery life.
The extension (in Advanced) has a Not Reachable destination that defaults to voicemail. You can set this to a destination that forwards to the smartphone as a normal voice call. If you need to ensure that calls never go to mobile voicemail, use Confirm Calls.
Simpler than the above, just setting up Follow Me may do what you need.
In my experience, sending incoming calls to a mobile VoIP app (even if it’s always reachable) does not make for a good experience, because the Wi-Fi or cellular data connection will sometimes be inadequate for acceptable voice quality. If I can’t answer at my desk or an alternative on-site device, the call gets forwarded as a normal cellular voice call. An exception is traveling where regulations limit me to a ‘data only’ SIM.
I agree, but with that agreement want to reinforce that this system is flexible on purpose. You may find this data-only approach works for you most of the time, but there are many other solutions that may be helpful. The “unreachable” solution (for example) is one that I’ve used. Others have tried one or more of several approaches (some of which @Stewart1 touched on).
One of the advantages of the method you are using is that all calls come through your PBX and are identified with your Caller ID. This is often important for reasons like fiduciary responsibilities and privacy concerns. Using the “Not available” destination to the cell phone requires discipline for personal cell numbers to not get “out in the wild”.
There are so many different approaches that work, giving us a system that can overcome many different foibles of real life. Cell phone extensions, DISA call-back systems, and other approaches can get you where you want to go.