I assume you're referring to outbound calls FROM your cellphone, and getting billed by cellco (AirTouch, for instance) on such incompleted calls from your cellular phone.
But the question here, is how cellcos will "supervise back" to the originating carrier (whether landline or wireless), on calls to an "unavailable at the moment" (or roamed out of the service area) cellphone numbers! In theory, the calling party (regardless of whether they are landline, cellular, calling card, coin, etc) should NOT have to pay for reaching such an "unavailable or roamed out of the service area" recordings when calling TO a cellphone. But even with SS7, many cellcos will cause a charge condition back to the calling party, though this won't happen in all cases. And it is not consistant even within the same cellco. Some of their switches will cause a charge condition back to the calling party, some of their switches will NOT charge for the call, within the same cellular provider!