A couple of points: 1) God did not ask, He commanded. 2) He commanded by direct revelation. There was no question in Abram's mind that it was God who was talking to him, and there was no doubt as to what it was that God wanted him to do.
It is much different when we do not have direct revelation to tell us just where and how God wants us to do "something."
I think a lot of people “go” on missions who aren’t called. They think they have to go and they do more harm than good.
I know of others how are called to mission work and do go and are right where they are supposed to be.
Is there direct revelation? Probably not. But God still whispers to many and tells them exactly what they should be doing.