He could have used any modern technology mobiles why a flip phone?
That flip phone was originally sent to Tony by Captain America, at the end of Captain America: Civil War.
Per the events of Captain America: Civil War, the Avengers had broken up as a whole, and Captain America was considered by the government to be a fugitive. Towards the end of the film, Capt. sends this phone to Tony (delivered by Stan Lee), with a note that included the following at the end:
“So no matter what, I promise you, if you need us, if you need me, I’ll be there.”
That being said, Tony had nothing to do with the selection of the phone because it was sent to Tony by Captain America, to serve as a hotline for if/when things get really bad.
It's a basic flip phone because they're arguably more durable and less vulnerable to malfunctioning than the average smartphone, and the battery perhaps lasts longer too. Also, if the only needed function of the phone is to place a call - and it is - then, Captain America didn't need to get a smartphone (or equivalent).
It could be argued though, that..
since Captain America's phone # is clearly visible to Tony (and the audience at one point.. 615-sth-sth), then, Tony could have just saved Capt's number into his own system, instead of carrying the flip phone around (unless there's special security features also applied to the phone).