Why don't you try calling the other number he's calling from ?!?!? Could be you're calling his cell and he's calling back from a landline to cut his cell costs - I do that all the time because I have a very basic plan with very little included airtime.
This could be the case. Only thing that remotely makes sense. Thanks.
My cell is for my business but I still give the number out to pretty much anyone because I always have it with me. But I have a few friends who are chatterbox's and if I see their name or number pop up I don't answer but will call them back from out home landline. Have you asked the guy why he does that ?!?!?