When I have to tell someone no, I pick up the phone. I hate talking on the phone, but I do it anyway.
When you're answering no to someone, you're disappointing them even if just a little bit. So you owe it to them to talk instead of sending an email. It's the polite thing to do.
But there are two other reasons, selfish reasons, for making the call. First, you get immediate feedback on how they took the news. If they're upset then you can do damage control straight away. And at least you know! And second, delivering bad news directly and respectfully is an important skill to develop. We can all use the practice. And it's never as bad as I think it will be.