[frankus]

I imagine the requirement to be the default phone app is in part to prevent rogue apps from colluding with rogue expensive-toll-services to place calls to them without the user's authorization.

[bobbiechen]

It's not hard for apps to commit toll fraud - all it takes is sending SMS or phone calls programmatically. Here's the Google Play page on billing fraud: https://developers.google.com/android/play-protect/phacatego... , and a writeup about malware used for WAP billing fraud: https://www.humansecurity.com/learn/blog/satori-perpectives-... .

[viraptor]

I don't think that applies here. You get billed for the connection. But you can't send DTMF without being already connected (not in the GSM world anyway), so this can't affect the call billing.