We have currently built a new SMS route using the Telerivet SMS gateway that allows any android phone to act as an incoming/going SMS gateway. It works great - except that we are also experiencing this problem:
Any ticket not originated by SMS cannot be sent an SMS via the Zammad GUI or the API (for that matter).
We are happy to push our contribution as a Pull request - but if would be great to solve this issue first @martini ?
Bart (Farm Radio International)