Hmm, this is a tough one but I have a few thoughts you may want to consider.
First, most guys don't like to talk on the phone, at least not the way that women do. Personally I hate talking on the phone and I know a lot of guys feel the same way. So, it's possible that when you call without any particular reason, he's just not into it. The obvious problem here is that it's a long distance relationship and the phone is the main communication tool.
Second, I think you two would have to have made a pretty strong connection for this thing to work. Guys get pretty horny and waiting months at a time just to see their long distance girlfriend in person is really difficult. I'd say unless he is drop dead in love with you, then you may want to consider dissolving the relationship or perhaps keeping it on "just friends" terms until future logistics are more convenient.
Just my thoughts, hope it helps.