What should I do? My girlfriend's ex-boyfriend wants to get back together with her?

You sound WAY better than Maurice. I'm not sure what you did was right because that really is up to Laura, but you seem like a much finer young man. Wait and see if he calls again. Because you said, "I'LL call you back," he will be waiting for her, rather than calling her. You just sit tight and relax, if she has brains like you said she would pick you. Also, since she has no idea Maurice called, she won't call him back like he's expecting. But he doesn't know that you two are dating? it's best to make it more public. Unless of course you don't want to. However, he will probably never get the "callback" and Laura will choose you--hopefully. Good luck!