Each VOIP line using full a quality telephone call will use up 90 Kbps but you can also lower the quality if bandwidth is an issue to either 60 kbps or 30 kbps. The quality will be lower but you can have more simultaneous calls going.
I would recommend for the 10 lines at least a 1 MBit line and if you are going to be using it for web browsing as well I would go with a 10 MBit line.
But as I said before if it is an issue you can dumb it down a bit. You should also have equipment where QOS or quality of service can be implemented as you want to prioritize your voice traffic ahead of your data traffic. If it takes an extra second to download a file it is ok but if your voice packets are delayed the call can sound horrible.