correction of a java class in android app

I have a java class in my android app, i want to ,modify it when I press the ok button in a dialog fragment it checks the entered phone number if the number is registered it should run another fragment if not it should go in the registration fragment. I have code and it is working checking the number with server and return the details of contact but the problem is i can not switch the fragments i think because they use main thread. it is urgent