Every line, every call, uses 80 Kbits/second of bandwidth to send audio out to the outbound caller while using 80 Kbits/second of bandwidth inbound to the Tel-One extension. This is effectively 156Kbits per second for each call.
You need enough bandwidth to support the number of simultaneous calls you need. If you want to make ten calls at the same time, you should have a 1.5Mbit Internet connection in both directions. If you have a connection with 1.5Mbit down and 768Kbit up, you can only make five simultaneous calls, because the uplink speed is the limiting factor.
You phone quality and bandwidth requirements will also vary if you have many computers using the same internet connection.