Hello, Newbie here , I want to make a voip GSM gateway for international calls . I am planning on using RasPBX, and I have ordered a 3G usb modem to use with it, however 3G network has been completely shutdown in the country I live in. Would I need to get a 4G usb modem, or will a 3G modem still work? There does not seem to be a lot information online regarding this issue and Voip in general.
I would assume if the networks are shutdown you will not be able to use your 3G devices and would need 4G or 5G devices to connect now.
GSM is 2G!
2G, 3G an 4G are very different technologies (time and frequency divisions multiplexing of gaussian minimum shift keying; code division spread spectrum; and coherent orthogonal frequency division multiplexing). A device designed for just one of them would not be able to cope with the others.
Modern radios tend to be software defined, so it is possible the same hardware could work, with different firmware, although I think it is more likely that the firmware would handle multiple generations, and the device would be advertised as not just one standad.