Multiple codecs ensure precise speech transmission based on your internet bandwidth and hardware.
When a call is made over the internet using a Voice over IP (VoIP), the voice has to be encoded into digital packets so it could be transmitted electronically. For any call, the encoding and decoding take place simultaneously (encoding is used to send the digital data and decoding is used to receive the digital data from the other end). Codecs are used for compressing data so it could be transmitted faster, providing better VoIP calling experience.
G.711u and G.711a: These codecs provide accurate transmission of speech and require very low processor requirements. For seamless and uninterrupted calling, you need at least a bandwidth of 128kpbs.
G.729: If you are looking for more accurate bandwidth utilization or if your internet speed is poor, you can switch to G.729 Codec.
GSM: Provides highest compression ratio. It's the same encoding that's being used in GSM cell phones.
The difference lies within the equipment being used. If you are living in the U.S., you are probably going to get the VoIP equipment that's compatible with G.711u. But for anywhere outside the U.S., you will be using the G.711a codec.
No, switching between codes is absolutely free on AXvoice.