I implemented InCallService and it works fine as I intended. When someone calls my phone, My app can answer, talk, and hang up. Of course, I understand that it is difficult to access audio streaming from a programming perspective due to Android security, and it does not matter. Simply hearing and speaking myself is enough.
However, if the call is made from a smartphone, I do not hear any sound even when I press the keypad. When my landline calls me and my app answers, I can hear the DTMF sound just fine. Perhaps one of the subsystems decided it was a mistake and filtered it out.
Is there a way to remove this filtering? It doesn't matter whether DTMF infomations are delivered to my app or not. Just being able to hear it with my own ears is enough.