Current Gammu versioning scheme confuses people a lot. For some time 1.x.0 versions are stable and 1.x.y are testing. However when somebody sees 1.10.5, he things this is updated version of Gammu 1.10.0. So it's probably time to move to something more obvious starting from next development cycle.
To stay as much compatible as possible with current scheme, I thing about minor change - so start testing releases on something like 1.11.90. I thing this is also quite often used (I've seen such versions in PyGTK in past) and nobody will confuse it with patch releases. Or should I switch to something completely different? I currently do not consider 1.11.0-rc1 as I'd like to keep version only numeric.