Further to my exploration of VoIP .....
SIP-based VoIP is an open standard, so unlike Skype (which is proprietary) many people create new services for SIP ....... it has that slightly raw but exciting feeling like the early days of HTTP (hell, I remember that !!).
One useful service I found recently is an equivalent of DNS for Phone Numbers, called ENUM.
You register your landline phone number, pointing it to your VoIP number. If someone calls your landline phone number using VoIP, via an ENUM lookup service, then that call will be routed for free to the VoIP number. (If the number is not registered, it would go out via the VoIP-->PTSN Gateway of your normal outbound provider.)
No new phone numbers to give out, no complicated dial prefixes to call from one VSP to another, it's great !!!
There is a free registrar at e164.org. I registered my landline number with them. During the registration process, their phonebot calls you to have you validate that you own that number.
There are two free ENUM lookup services that I know of : SIP Broker and Voxalot.
Voxalot has the advantage of allowing you to setup your preferred outbound providers with them, along with a set of dial plans, so if the number you are calling does not have a VoIP equivalent, than your call goes via the cheap (or free) provider of your choice.
I now have free phone calls to local, national and around 20 international destinations (the useful ones to me) using VoipCheap, while staying registered with VoipFone for incoming calls where they offer better services but higher call charges.
This so Rocks !!!