Am I overreacting feeling insecure if my boyfriend doesn’t call when he said he would and the next day I find out he was up until 5 am socialising with his ex and her boyfriend?

No , you are not overreacting. It was not very nice for your boyfriend to not answer your calls. I would be a little upset as well. It is natural to feel insecure. It would be best to openly speak with your boyfriend and see how he feels about his ex girlfriend and ask him if he still has feelings for her. Take care of yourself and make sure you are not being taken advantage of. Sometimes men do not know how to express deep feelings and it may take some strong questioning on your part. You do not want to be choice number 2 especially if you are calling yourselves boyfriend and girlfriend.