I just recently did it for the first time... just think back when you had sex with him before and talk about it... "First I'd unbuckle your belt with my teeth" kind of thing!
Have funs with it, he knows you've never had phone sex before so he probably will understand if it's a bit rocky.
Just fantasize... what would you LOVE to do to him? Tell him you're wearing something kinky, (be best if you really were wearing it!) and go from there. Be very discriptive with EVERYTHING... and play with yourself while doing it... he should be too!
It is great, you just have to get really into it!