I stream an mp3 radio station, have a broadband phone, and run Tradestation with about two dozen symbols and IB's TWS with same symbols. Doesn't seem to effect it at all.
It depends illiquid. But you can work it out fairly easily. I believe most phones use much less than 64000 bps each way (more like 15,000bps so that you can just use them with dial modems and internet connections). Even 64,000bps is much less than the 7,000,000 bps I get on my cable and 1,500,000 bps you might get on a poorer broadband connection. Thats 1-4% and wont affect your trading datastream. You are more likely to have contention over processor load between trading apps (charting particularly) and the voice processor so if you had a very slow pc you might suffer from the interaction (press alt ctrl del to bring up the task manager and look at the cpu load of differnt processes).
I've got Vonage, and it's the best move I've ever made.....works great, and cheap as anything....... Never interfered with my connection speeds
Using Time Warner cable phone and it seems not to affect my quotes at all. Pretty clean sound as well.