Recently a friend asked me about which VOIP solution is best this is what I told him.
"Skype is the easier to setup it handles NAT traversal very well, the handsets we use for Skype (<£30) are USB so need to be plugged into a PC to work. I believe you can now get Skype Adaptors for normal analog phones and Skype phones that don't need a PC to work, I don't have any experience of these and they will be more expensive ~£100.
VOIPtalk offer SIP/IAX2 services, the handsets that support these protocols start at £50 but you should spend at least £80 to get a half decent one. You will need a CAT5 socket to connect them to your LAN although you could use WiFi. If you want to have several phones in your house with each room acting as an extension then you should think about installing Linux/Asterisk onto an old PC and run your own mini PBX. This will take a lot more setting up and the learning curve is a lot steeper. You pay a one-off fee of £10 for an inbound 0845 number and buy credit in £5-£50 chunks, you can set your account to auto top-up to save you the hassle of topping up via their website. If you go the VOIPTALK route make sure your Broadband Router has QoS functionality otherwise you won't be able to hear people when you are downloading large files. It will also need port forwarding and I would recommend using IAX2 over SIP as it only requires one port to be forwarded.
Some (most) routers will not allow SIP to traverse NAT.
In terms of call quality / reliability Skype has the edge in Internet=>Internet calls (which are also free) and Voiptalk has the edge in Internet=>Landline/Mobile calls.
Re call charges, I don't think there is a lot in it but you can checkout their respective websites.
I'm sure your aware that more and more ISP's are starting to offer VOIP services so you might be able to get what your looking for that way."