Let's say a businessses purchases a sufficently fast internet connection (such as 60Mbps cable connection) and VOIP quality is pretty good for the most part. The only thing I would be worried about is a user initiating a file download and saturating the line. Would it be possible to restrict bandwidth on a per-port (logical port) basis. For example 5 Mbps is reserved for SIP traffic and the maximum a file could download at is 55Mbps. The setup would look like:
FreePBX --------> Cisco Router -----> Internet (SIP provider is here)
Users ----------> /\