If you already have an internet connection, then you have the ability to connect a special type of phone (a VOIP - voice over Internet Protocol) to either your computers usb port, or direct to your internet hardware e.g. broadband router.
Generally you will be able to make FREE phone calls (even worldwide), where the recipient also has a VOIP phone/setup, or calls at drastically reduced rates when calling landlines, and even other mobiles. But special 'UK' numbers such as 999, 0845, 0870 etc will not work through any type of phone/network unless you are actually in the UK.
Skype has been one of the leading voip providers, but now other services such as www.voipbuster.com and www.voipstunt.com are competing.
With the latter two services, and in conjunction with a special gateway box (about €70) you can connect a normal phone to your internet phone, and make completely FREE phone calls to landlines in other countries. A PC doesn't need to be on!
In addition, if you buy a Spanish landline number (you can do this even if you don't have a landline) then friends can call that number from a UK landline at a cost of only 0.5p / min. So you can call them FREE, and they can call you for almost nothing. Contact info @ spanishpropertygateway.com for more details