Boyfriend and calls...?

My bf and me are approaching our 5 years, everything is okay when we see each other.. days at a time. Then when we dont I feel like he's completely disappeared. He'll call and say I'll call you later.. and not, he rarely picks up his phone and takes a long time to call me back -- should this bother me? Or should I just let it slide... I've talked to him about it before he says he's just forgetful...but is he? I don't know if it's worth getting mad over it.. because I don't want to create unnecessary tension in our relationship. I try to focus on my own priorities... but some how this bothers me... what should I do?