This might be of use to you if you are still interested in development.

[dust0vich]

https://github.com/snom-project/DECTMessagingDemonstrator/blob/master/app/doc/Feature%20Specification%20-%20Text%20message%20and%20alarm%20key_version.pdf

[SmithChart]

Thanks for the pointer. This looks interesting!

I have stated this project on Christmas vacation some years ago... But the next holiday will probably come :-)
I'll leave this issue open to remind myself.