A telephone terminal executes voice communication with a partner telephone terminal by using a selected codec program to code and decode voice signals. Both telephone terminals have facilities for storing codec programs and transferring codec programs over a communication network. During a voice call, one of the two telephone terminals may transfer a codec program to the other telephone terminal, and the call may then proceed by use of the transferred codec program. This enables voice communication to take place with the best codec program that either telephone terminal can provide, even if the two telephone terminals are associated with different communication systems.