What API gateways provide @ vtext.com addresses, etc.?
Guaranteed delivery.
SMS email gateways often drop messages, especially if you send multiple messages through them. They may even block you if you exceed the (undocumented, completely arbitrary) bid. In practice, I would not expect more than 90% success sending emails through the email gateway of cellular providers.
In addition, in order to properly use the gateways, you ask your users about your provider, maintain a database of provider gateway addresses (which, as you know, have been changed in the past). Remember also that most of the world has number portability; this means that you will need to constantly monitor delivery failures (from gateways that even bother to send them), and ask users to update their provider.