This is an issue that bugs me--spam sent to cell phone as text messages that the recipient must pay for, and the lack of responsiveness from the cell phone carriers. I got a few such spams that I had to pay for and my only option was to turn off texting capability; an option I think is wrong but I had no other reasonable choice.
An article in the Phila Inqr discusses this further. See:
Can't the cell phone carriers use ANI to trace back to the offending source? If the caller spoofed their callback number (as I suspect they do), couldn't they be prosecuted for that?