After some years I had a SD card failure. The card I had used was already much used and old.

I used this occasion to give the whole system an upgrade. Debian now version 10 (which was the current version at that time), a raspi zero W instead of the B+ and a printed custom housing.

Of course, the software I used had also undergone some upgrades in the meantime (in particular gammu). Therefore, I had to amend the config file, too.

Since then I has been running without any big problems. Except that gammu seems to hang every now and then. Therefore I restart gammu once a week and to be on the complete safe side I restart the system once a month. This seemed to solve all troubles.

The whole software including the config files and crontab entries of both root and the user can be found on codeberg together with a translation of how it works into English.