During the summer 2008, Sim4travel was offering free incoming calls in Morocco. It worked well for outgoing calls, but not for the incoming ones: people that wanted to call me had to repeat it many times, because they were directly connected to the voicemail for at least 50% of the calls. But despite of that, it was a good and relatively cheap solution.
And during the same summer,
Prion said here that UM+ (R.I.P.) was also offering free incoming calls in Morocco.
At this time, I find no alternative