My rule of thumb is that if someone calls, but doesn't leave a message, then the call couldn't possibly have been that urgent or important to that person. Leave a message saying that unless he's going to explain where he's been the last couple weeks, then he shouldn't bother calling back. Give him a deadline, after which you won't return his calls no matter what he says.
He might just be one of those losers who, rather than come out and say he wants to end an intimate relationship, just stops calling instead. I've met a few of those.