A codec, which stands for coder-decoder, converts an audio signal (your voice) into a compressed digital stream for transmission (VoIP) and then back into an uncompressed audio signal for replay. It’s the essence of VoIP. Codecs vary in sound quality, bandwidth requirements and/or computational requirements. Each service, program, phone or gateway will typically support several different codecs, and when talking to each other they have to negotiate which codec they will use.
At M12 Solutions we typically use the G.711 Codec.