I would say that "85%" coverage is an exaggeration. While most significant towns have cell phone facilities, once out on the open road between towns, or up in the mountains, you are usually out of range.
It's the poor south that has the least coverage, but that's where we go is it not.
Morocco is not going to provide 100% coverage for areas that their own population rarely enter are they?
Of course as Fireboomer says, you may get a signal on a hilltop from a nearby mast, don't rely on the fact. And the southern desert is signal-less apart from near Taouz I found.