I have an ATT iPhone, but I thought there was an option on phones with a Verizon SIM in it to turn off CDMA international roaming. I thought they added that to deal with the situation where there was a local CDMA carrier (who you couldn't roam on) and a GSM partner which you could roam on. In that situation my understanding was that the iPhone never saw the GSM partner. The switch was designed for that purpose.