A

AT&T Landline Texting API

REST discovered

Details
Docs
https://developer.att.com/landline-texting/docs
Authentication
AT&T Landline Texting session key bearer discovered

Buy AT&T Landline Texting through your AT&T account team; the docs say to contact your AT&T account manager to get started. After ownership verification of the text-enabled phone number, AT&T emails you the session key for the account. If you already have an account but cannot obtain session keys, the docs direct you to support.