The "plafond" is a limit, but "level" sounds better to me. What seems to be current usage, however, is "prepaid service plans"...minutes or money being possible plan options.

Sample text:
Speak Out is a prepaid phone service, so the phones come loaded with a finite number of minutes that can later be renewed. Consumers aren't required to sign a service contract. About 11 percent of all U.S. cell phone users are currently on prepaid service plans.

Taken from: http://news.zdnet.com/2100-9584_22-5201062.html