"More than two decades after the first digital roamed voice calls, 4G has caught up with what GSM was invented for"
Err.. 3G voice? Also 3G data rates have steadily increased over time with features such as carrier aggregation.
As to voice quality the same codecs are available.
Two encoding techniques? Do you by any chance mean FDD/TDD? That is protocol not encoding.
Frequencies - There are specific TDD bands (33-44) so a TDD capable phone will be able to talk to a TDD network. I would have to check if there are subsets for different regions the same as FDD.