Use APKPure App
Get 15.5 legacy app old version APK for Android
App for 3CX Phone system Server Version 15.5. Will be discontinued early 2020
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