A lot of modern handsets are multi-mode and can switch between CDMA and GSM once unlocked. But yeah ultimately comes down to needing to select a GSM carrier.
My Pixel 3 can do simultaneous voice and data, because it’s in Australia connecting to GSM based networks.
If it were connecting through CDMA, it could not. CDMA has always been the minority worldwide[0], and is phasing out completely with 4G.
If you’re on a CDMA-using carrier (Eg Verizon) but usually have 4G coverage, you should be able to get simultaneous voice+data by enabling ‘advanced calling’ in the phone’s mobile network settings.
