@Sorby
Sorry to dissapoint you, but I don't see that feature implemented too soon, and that's because we tried it already and it was available for few months then we removed it; the main reason: sometimes those extensions remained stuck in "DND-In GSM call" indefinitely, because the GSM call lasted longer than 3CX client' maximum allowed background time, so the OS was killing the client. With its status remaining in DND, even in today's PUSH environment, no incoming call would be allowed by the PBX to wake app up back.
As I mentioned - this was the main reason. Add to this that an application running in the backround 24 hours per day 7 days per week it's a hell of resources consumer. There were also reasons related to network rights, PBX restarting, user deleting/switching account, etc, which, bottom line, made this concept very unreliable.
I'm assuming that, despite my explanation, you're still keen to see it implemented - in this case, my advice is to post it to the Ideas section (as far as I remember, there is already there posted by somebody before you quite long time ago).
Regards
vali
LATER EDIT:
Oh, it was you then too, 2017
https://www.3cx.com/community/threads/mobile-phone-availability-connector-to-3cx.49010/