Amb,
It depends on many factors such as what type of compression you are going to use for VoIP, how many calls you want to support and what else you will be doing on that line.
A typical call using G.723 requires between 15 and 18kb per call, per direction (transmit and receive). So that comes to between 30 and 36kb total for the call. Then take in to account any web browsing or other tasks that you may do and that will decrease the performance on the line.