> I understand the advantages of supporting SMS MT+MO and network auth, but IMO it's dangerous to support all these in our v1 API if:
1) the clients we implement and release don't support these alternative mechanisms

The client is going to support both SMS MT and SMS MT+MO for MVP.

> 2) the server has to resort to heroics to determine which v1 clients support the alternative mechanisms, particularly with the requirement that the server determines the auth method in /register

If the client doesn't support the server recommended verification mechanism, it will fallback to other supported mechanism (SMS MT most likely). Note that `/register` doesn't enforce the usage of a verification method by giving a specific session token that is only valid for the recommended verification method.