It depends on what you're comfortable with. I'm not comfortable calling a customer, because I do not want them to have my phone number. That's why I have a shop on the internet. Plus, if you have all the information in a convo and something goes wrong, you've got a "paper trail" of proof regarding what you said and she said. Plus, if you take down info on a piece of paper during a call, you're far more likely to lose it, toss it, have someone ELSE toss it, etc.
Again, that's just me.