I picked up a bunch of cheap GSM modules on aliexpress. I got ten from one company and ten from another. I wanted to get enough that I could run a workshop and also if some are soldered wrong or just don’t work I wanted some backups. The 20 modules were nearly the same price as a single GSM module made specially for the Pi.
I will know in 14-20 days how they work out.
This might be helpful when the parts arrive:
http://www.academia.edu/15633281/GSM_GPRS_Modem_-_Neoway_M590_M590E
