My understanding is a valid warrant (which as someone else mentioned does not have to be a physical piece of paper) grants them immediate access and they don't

My understanding is a valid warrant (which as someone else mentioned does not have to be a physical piece of paper) grants them immediate access and they don't have to wait to verify. If you don't open the door, it will be opened for you by battering ram - demanding you open it is more of a courtesy. Think about it: say someone has a bunch of drugs, you wouldn't want to give them time to start flushing it down the toilet while they call up and verify the warrant is valid. You kick the door in, arrest them, and secure the scene for forensics and evidence collection.