If I recall correctly from my base station programming days, GPRS is
the bearer channel for SMS, so if SMS isn't working, it will almost
certainly be because a GPRS channel cannot be opened. I think
Verizon's GPRS only works in areas where it can be executed with lower
power consumption, which means being close to the ground radios. I'm
very curious whether other service providers can do SMS at altitude
(or any other packet-switched service).