Internet to internet (same call provider) = free
Internet to internet (different call provider) = often impossible
Internet to real phone = requires purchased call credit (on systems that support outgoing PSTN)
Real phone to internet = requires subscription to incoming number, on systems which support it, the incoming number can be in another country if you wish.
There are services other than Skype, which do not abuse your connection for forwarding traffic.
http://www.voipcheap.co.uk/en/index.html
Skype's level of penetration, firewall workaround and sheer momentum make it hard to go against for internet to internet calls, the alternatives may be a better choice for internet to and from phone.