How much bandwidth does my network need to support a VoIP phone system?

The minimum recommended bandwidth is 100 kbps up and down. Keep in mind the more call paths you want the more it will use your bandwidth. 

2 common audio codecs and their approximate bandwidth usage per call path are G711: 85kb per call path / G729: 30kb per call path

Not sure about your speed? Test it here! www.speedtest.net