If your cellphone use a microship the only company that use GSM tecnology is Orange, if you have the roaming service activated you phone maybe doesn't has the right PRL. the other companies use CDMA Tecnology
If don't want to use the roaming service you have the opticon to activate it with pre pay plan, the activation is free, you only need to know the telephone activation code, and program the PRL that cost about 50.00 pesos and that it.