Have a look on ebay - lots of people selling cheap old Tesco Internet Phones still in their packets with "£5 free calls", which is misleading. Using a softphone you can even get away with a mega-cheap headset, I think I picked a couple up for 99p each that I used on mobile broadband with VOIP accounts.
I've put a Camrivox UTA151 on my network at home, with a standard DECT phone plugged in. Works fine.
You don't need to spend a lot to get a working VOIP system.
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Virgin Cable (L)