Free Calling from USA to UK?

I'm looking for something I can use for free calling or texting from the US to the UK. It can be an iPhone app or computer--either works. I already have Skype installed, but I'm looking for something that won't require the other person to also have it installed.

If you are having skype and the other one has a headphone it will work… Headphone is a simple thing everyone has it (most people)