What determines VoIP Phones' quality?

[RossCraig]

I am thinking to buy a VoIP telephone to replace my current landline phone. I wonder what are the things I should consider before buying one? I googled a bit and found that all the phones have basically the same functionality except for the extreme high end ones.

[Graeme]

Basically your internet speed. If you have an old skewl 56k modem and your trying to run VOIP then youd be pretty screwed. However, if you have a 2mb connection then you will get fast speeds and be able to get better quality.