Basically, you are both using air time so you are both being charged for it. Some companies offer in-network calling plans so that doesn't happen. You would have to check with your provider.
Otherwise, if you call a land line with your cell phone, only you will be charged for the minutes. I have a cell phone but I rarely use it and I don't give out the number so people have to call me at home.
Cell phones are convenient, but they sure are expensive!