First of all, only begin if you're feeling aroused as well. There's no doubt he'll become aroused, but neither of you will enjoy it if you're not both into it at the moment.
The next time you're talking late at night, tell him you're sad to go to bed because you'll be so lonely without him. Tell him you wish he was there with you holding you, snuggling up to you, running his hands all over your body. Up your legs...up your stomach...over your breasts. Just start very slowly and kind of tease him along. Don't just launch into it like it's a speech you've got prepared. Let inspiration strike you along the way - if you think of something to say, go with it. Play off of what he's giving you. If he just starts responding with "oh yeah" and "uh huh", then there's not much you can do with that. Encourage him to be descriptive. It may feel weird to one or both of you at first, but if you've already been sleeping together, perhaps some of that will either be gone or wear off quickly. I guarantee you that the next time you're physically intimate with each other, it will be greatly improved: communication, especially about what arouses you and what you want to do to the other person and have that person do to you, always improves a sex life and strengthens it.
If it's going well on the phone, then snap a picture of yourself as he's describing a part of you. Then send it to him. Tease him with the pictures. Make him feel like each picture is another article of clothing he's peeling off of your body. Don't just straight away send him a picture of your boobs. (Although that would certainly be a lucky gift for him.)
The important thing is to be relaxed and have fun! Oh, and make sure you are aware of your cell phone plan: make sure you both have enough minutes and text messages to do this without paying an arm and a leg for it!