So basically, the director of the music video has stated Adele rocks a flip phone in 'Hello' because he doesn't like filming modern day technology, as he feels it takes people out of the story and way too much into reality (as opposed to having a glaring technological anachronism but w/e). This is also why she uses not just a rotary phone, but one of those pop-up phone number indexes straight out of the 90's. According to Dolan: "If you see an iPhone or a Toyota in a movie, they're anti-narrative", so everybody in the next Garry Marshall rom-com better be talking on a brick mobile phone and driving a steam engine automobile. Making a little more sense, he says that the flip phone is a connection to the past - she's not actually in the past though, it's a metaphor for not being able to let go, like how your mom won't give up her damn Nokia since she can just check her email when she gets home and only really needs it so people can reach her when she's out. Dolan could have just said "shit takes place in 2004", but there you go.
Flip phone post!