So, fix the damn cellphone towers. If a jailbroken cellphone can crash them, then surely a malicious person with a laptop and an antenna of the appropriate type could also crash them.
If the unwashed masses suddenly gain access to such capabilities effortlessly then it would be a concern. Presently it is not, because smart phones have GSM subsystems carefully separated and ‘dumb’ phones have certain limitations built in. Behaving graciously is assumed a mobile station's role in the GSM system.
Recall the Nokia 1100 rumor eariler this year where underground allegedly gained acces to frimware memory map of certain series of this phone and its price skyrocketed on second hand market.