How much bandwidth do I need for VoIP?

Your connection speed (bandwidth) must be sufficiently high for the type of vocal codec you are using. Vocal codecs (or just "codec") are the standard means by which your IP Phone or ATA encodes your voice and transmits it across the Internet. The codec that is picked for a particular phone call is primarily determined by your ATA's configured preferences.

To use the best quality / highest bandwidth codecs (G711a/u) it's recommended a connection speed of no less than 90Kb/s in each direction (up and down).

A minimum connection of a 512/128 kbps  is recommended and should be used for a single VoIP line. Any lower and the quality can drop considerably, particularly if downloading or surfing the web at the same time.