This is known issue people are experience with CM9, Gummy, and AOKP. MotoBlur + Android 4.0.4 is based of AOKP inheriting its issues. Usually a restart or two fixes the issue, but it is somewhat random. I believe the issue involves bluetooth but I guess no one really knows. I heard toggling bluetooth sometimes fixes it as well.
However you say it doesn't even ring, I have never had it not ring. But have experienced the rest.
Droid 1 - ChevyNo1 SS 5.6 1.0Ghz LV Kernel (retired)
Droid 3 - Gummy ICS 4.0.4 Modified Build 1.2.x built from source