Thanks to a certain Irish low-cost airline company, I'm in Morocco now and here is my feedback:
GSM coverage is surprisingly good even when climbing the Toubkal (max altitude: 4157m)
I bought a prepaid "Méditel Médijahiz" simcard (official price: 30dh, 2.5€ and Souq price: 20dh, 1.65€) which provides good coverage and excellent sound quality
With this Sim, I use a "Méditel Dawlia" scratch card: Calling western Europe landlines is for 2.5dh/minute, 0.208€ (peak period) or 1.75dh/minute, 0.145€ (after 8 p.m.).
Internet access is rare when trekking in the Atlas, so no VoIP, but that's not a problem: Sound quality is much better than any VoIP solution with these Méditel cards (a company mainly owned by Telefonica), and the scratch card is very easy to use: You just have to store 133*scratch_number on your phone, so you can dial directly once you're connected to the server, after very few seconds.
Thanks everybody for your help: I found much more reliable (and cheaper!) than Jersey cards