The issue was opened at end of 2008: https://issuetracker.google.com/issues/36906273 and there is no solution on Android. Well, not anymore, this guy found a workaround:

https://edm115.dev/blog/2025/01/22/how-to-send-dtmf-on-android

He used the Android Accessibility to get access the Android Telephony keypad and send DTMF sounds from there.

And he did it to add support to https://github.com/lifecompanionaac/lifecompanion a really interesting project for people with “special needs” that I never heard before!