If Steve and Sam had called Tony, Tony would probably have betrayed them
Considering Tony's arrogance throughout Civil War (and really, every film he's been in), this theory makes sense. Tony didn't care about the threat of the Winter Soldiers at the airport, so why would he care about them before the airport?
Note: Set in an AU where the Avengers reversed the Snap rather than give in to Tony's extortion and Thor went for the head. The U.N. abolished the Sokovia Accords quickly after Thanos's defeat in Wakanda. The stones automatically returned to their original universes after Bruce snapped.
***
Steve didn't know WHY he was standing in front of a particular brownstone. When Bruce told him that this was where the Time Stone was kept, Steve felt compelled to come.
The door opened and a cloak waved at Steve to come in. Steve walked in, and followed the fluttering cloak to a library where Dr. Strange waited.
Dr. Strange looked at Steve for a long moment. Then he said, "You made the right choice."
Steve asked, "What do you mean?"
Dr. Strange shrugged and said, "You remember what happened--EVERYTHING that happened. So do I. You think there was a point in time when you should have chosen a different path, don't you?"
Steve bit his lip, then nodded. He said, "I can't help thinking of that moment in the hideout where Sam and I talked about calling Tony. Maybe--maybe if we HAD, things would have turned out differently. And maybe we could have stopped Vision being injured, or your trip to Titan."
The sorcerer shook his head. He said, "You made the right choice. If you had trusted Stark, the universe would have been lost for good."
He gestured and a picture appeared in the air. In it, Tony was angrily shouting into a phone, then he slammed down the receiver. After glaring a moment, Tony picked up the phone again and made a call. Thaddeus Ross answered.
The image disappeared as Dr. Strange said, "Stark would have betrayed you. He didn't care about the possible threat, he cared about being obeyed. If you HAD trusted Stark, your entire team would have been imprisoned on the Raft. You would have escaped, but Ross would have killed Barnes and broken Maximoff. The Earth wouldn't have stood a chance, and no one would have been left to participate in the time heist."
Steve sighed and said, "Thank you. It's been tough for us who remember both timelines. We're still feeling the effects of those five years even if they no longer exist. It helps...to know that we did the right thing."
Dr. Strange nodded and said, "Glad to be of service."
Steve said, "The rest of us are thinking of getting together to...just talk and be with others who understand. Would you like to come?"
Dr. Strange smiled and said, "I think I would."