Firstly you can get away with not having an internet connection at all, but you will miss out on our backup and roaming service (although the data backup could be done over a dial-up line).

Your current ADSL modem will probably do, but if you are buying from scratch it is useful to find one that supports a number of features.

Firstly it's useful of it supports RIPv2 routing which will enable the modem to dynamically learn about the networks behind the voice server.

Secondly the modem should handle Quality of Service (QoS, often also called DiffServ). This ensures it gives priority to voice (and video) packets being sent up the line.

A normal ADSL connection with 512K upload speed will handle 7 simultaneous phone calls via the internet using alaw encoding (64Kbps each). You can very easily get more by opting to use GSM encoding which uses only 8K per channel but at a small loss in quality.

If you are using video phones it's another matter. 512K is just about OK for a single video call but true high definition will need around a 1Mbit/second upstream.