[dwaite]

Slight correction - you can't (or rather, shouldn't) override the SMS handling on an android phone.

Instead what an app like Signal does is request all the permissions it can from the SMS/MMS handling service of the phone - to read and send SMS entries, and to get events on an incoming SMS, and then request to be the default handler of the `sms` custom URI scheme.

But you can have any number of SMS clients at once. It is likely if Apple Messages ever came to Android, it would do the same thing - otherwise, the fallback behavior (when talking to an android user without the app installed, for example) would be sub-par.