Europe has adopted the "calling party pays" charging scenario. However America has always charged both ways. So if you call from your cell phone - assuming you live in the states - to mine (for example) you'll be charged airtime and land charges, and I will be charged at least airtime (possibly land charges as well if I am roaming). Most of these are covered by the included minutes of your plan however you are still being charged for the time.

There are a lot of tricky scenarios since a single call could go through multiple carriers - that's one of the marketing points for companies like Verizon and Sprint who own both the cellular infrastructure and the lines in the ground to connect between the switches or between the cellular and land network.