3G maintains a circuit switched network as I understand it, which is used primarily for voice call on that system, which the packet switched part mainly being used for data. 4G dropped the circuit switched part, so early 4G phones had to drop back to 3G to actually make a call, until VOIP was developed and rolled out for 4G networks.
I'm not sure if the Pyra's modem can be forced to drop back to 3G but I would largely suspect it's possible somehow.