Fallback to SMS only %XSMSFALLBACK

The proprietary %XSMSFALLBACK command sets the SMS only fallback functionality. With SMS only fallback, User Equipment (UE) immediately triggers a Tracking Area Update (TAU) request for SMS only when CS service registration fails with permanent cause. This ensures that SMS services are available as soon as possible after registration. SMS only and SMS only fallback are available only in Narrowband Internet of Things (NB-IoT). v2.0.x