Use APKPure App
Get 15.5 legacy app old version APK for Android
Aplikacja na system telefoniczny 3CX Wersja serwera 15.5. Zostanie wycofany na początku 2020 r
Use only if you are unable to update 3CX Server to Version 16
For use with Version 15.5 only
Can work on Android 6,7,8
Will have problems on Android 9
Does not work on Android 10
The proper app you should use is this:
https://play.google.com/store/apps/details?id=com.tcx.sipphone14