I recently came across a document that describes telephone CODECs on the Polycom site.

VoIP to 20 kHz:
Codec Choices for High Definition Voice Telephony (http://www.polycom.com/global/documents/whitepapers/codecs_white_paper.pdf)

Adding to the list of codec resources, here's a more VoIP-oriented comparison I came across:
http://www.ozvoip.com/voip-codecs/
along with a list of which common devices support which codec:
http://www.ozvoip.com/voip-codecs/devices/