Yes, fter the first generation network, carriers either chose GSM or CDMA standard for their 2G network. Both network are not compatible to each other. However, a WCDMA phone will be compatible with GSM network if it has a sim slot and has a GSM antenna.
GSM carrier frequencies are 900, 1800, 1900, and 2100 mHz. That is why iPhones are only capable with 2G network on T-Mobile’s network.