Originally Posted by Aston Lopes
I'm working on creating an app which will work as an offline voicemail
It depends about what you mean with "offline voicemail".
Recording a voicecall is always possible (until now) and there are already plenty of App doing that.
Auto-answer a voicecall, recording it and then show all recorded files is not exactly what I call "voicemail" because the caller could not understand what to do when the voicecall is established without any voice instruction.
I would like to play my custom voice message to the caller saying "hello, I'm currently busy but you can leave me a message after the BEEP". In this way the caller is informed about what to do.
I believe there's no need of root.I'm focusing for it to work on Oreo 8.1 (sdk27).If it works on Oreo then it may easily work on 5.0+ devices.
The main problem is that each manufacturer sets Android Audio Policy in a different way.
For example my App is currently working on my testing Samsung Galaxy Note 2 and Note 5 (Nougat 7.x) but it's not working on my new Sony Xperia ZX Premium (Oreo 8.0).
Sony seems to have excluded via hardware/software to play the voice message to the caller while there is an active voice call. This is the main problem: each manufacturer can change it's mind/configuration (in the future) and makes the App stop working more.