Recently we were able to get automatic SMS notification working using custom script - see below. (thanks to @Sagar_Bhogayata).
Since then we have implemented it for Sales Order and Delivery Note notifications. It seems to be working fine - including details such as order amount, etc.
Is it difficult to create a new SMS Notification doctype (dont know even if that is right term) - very similar to email notification - that executes enables / executes this script in backend?