Abram believes God but he asks how he will know that it is true. God tells him to prepare a sacrifice, then gives him a dream, then smokes the sacrifice to seal the deal. Wow.........
Then Sarai wants to help God out and presents Hagar to be a surrogate. Then Hagar gets pregnant and Sarai hates her. Isn't that the way we are! Don't be trying to help God out! He has a plan and will orchestrate things just fine!
Anyway, Ishmael is born and when God tells Abraham when he is 100 years old he and Sarah will have a child when she is 90, he falls on the ground laughing. Don't be ridiculous God - well, God can do anything He wants, anytime. He argues with God saying Ishmael will be the son and God says, nope, Sarah will have a son and call his name Isaac.
Notice that Abram and Sarai got the name change here.
Also, circumcision began at this time.