He is asking you to give him a telephone call when you are available. I wonder why he didnt just ask for your number and ask if it was ok for him to phone you.
The only time a guy has ever told me "call me", he's been on the fence between myself and some other woman but not man enough to make a choice. Basically, to the woman on the receiving end, "call me" translates into "You're better than a kick in the teeth but I don't know if I want you. If you call and I feel like it, I may answer, but maybe not if someone else comes along between now and then". Guys say they want women who don't play games and they like to think they're pretty simple, but they're so confusing.