在+v. means be doing. eg: 我在学习。I'm studing. 去+v. means go to do sth. eg: 我去学习。 I'll go to study. 去is a verb which means to go, while 在 is a preposition which has some grammar meanings like be doing sth. or at some places.

1.I will go and call him. 2.I'm calling him. 在 means sth is going on. 去 means "will be"Or "going to be".(we also use 去 as a verb ,the same as "go")

我去叫他→I am going to call in him 我在叫他→ I'm calling in him