Frequently Asked Questions (FAQs):Choice of Codec

VoIP: Choice of Codec

The choice of codec that you make can have a large effect on your VoIP communications.

To reduce the bandwidth costs of voice transmission, a variety of different codecs are in use.

FreeVirtualNumbers support the following Codecs on their Asterisk based VoIP network:

G711a/law (FreeVirtualNumbers Preferred Codec)
G711u/law
G729a/b

In Digital PSTN networks the G711 codec is typically used. This is widely thought to be the best quality Codec to use. FreeVirtualNumbers recommend this codec as we use this codec internally and when we pass the voice traffic to our PSTN gateway this also uses the G711 codec. Using this codec reduces the amount of coding and decoding of the voice traffic across multiple networks.

When selecting your Codec you need to take into account the amount of bandwidth each codec uses.

G711 codecs are the most expensive when it comes to bandwidth, using on average approx 80kbits/sec for each leg of the Voice traffic. (up and down both use 80kbits/sec each maximum)

G729 codecs use on average approx 32kbits/sec making this codec ideal if you are having bandwith issues. (in order to use this codec you must request it is enabled on your FreeVirtualNumbers account)


If you are using the Single Line account you can use any codec that the originating and destination devices can support.