I have the same issue. I can indeed answer, 3cx call gets moh, terminate cell call and go back to 3cx on hold. However... It sucks. Prevents me from having 3cx calls on cell phone with App.
Is this a bug, or a feature? Can't really figure it out with my forum/google and 3cx documentation searches.
Device Info
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Phone Model: [Xiaomi mi 9 / One+6]
Firmware Version: [android 10 miui 11 / android 9]
App Version: [16.2.0.57]
Additional details
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Handset: [3cx app]
Connection: [4G / WIFI]
Server Information: 3CX Server installation details, Internal, External, network info etc
Server OS: [Debian on Lightsail (PBX Express)]
Server version: 16.0.3 update 676