Tony Stark aka Iron Man is one of the richest character in MCU. Each time he shows up in a movie he uses high tech armours and other equipments.

In Avengers: Infinity War he uses nanotechnology in his armour. When Banner told Tony to call Captain aka Steve Rogers, he takes a flip phone to call.

He could have used any modern technology mobiles why a flip phone? What is the significance of that in the movie??

Flip Phone