Memes

They remember at time when we weren't all within reach of our own personal phone line 24/7. During that forgotten time, they were mostly children and expected to answer the landline and play the respectful secretary for the family. Sure, you MIGHT call someone's house if you cared or dared to run the gauntlet of dealing with whomever answered the home phone and it wasn't so private that you'd risk someone listening in from another room of your house or theirs. Party lines were even still a thing in some places. You could listen in to wireless handset phone with a baby monitor. Phone conversations carried a lot of emotion baggage.

The dotcom bubble burst just after we all got cell phones. As a result of this quirk of timing, most millennials grew up socializing a lot with people remotely via text based conversations over the Internet using things like Bulletin Board Services/Forums, IRC, ICQ, newsgroups, etc. These were free and far from the prying eyes of parents or easily hidden. But, that would have all been done at the home or school computer just like the landline (usually sharing the same literal line), not a thing you carried with you.

Millennials spent vast oceans of time being completely and utterly unreachable unless physically present and together, learning to converse face to face or in paragraphs of text from a box at home. Even emojis were text. Images were slow, small, and low quality, so the memes were rare and crafted with care.

When millennials got their first phone, it would have been likely for most that they'd most often be used by parents checking in. Cell phones were still mostly an in case of emergency type communication device, not your daily driver. That battery was limited and charging was slow. Even though text messages of the time carried a stiff financial cost, millennials stuck in class could converse by tapping out messages on the phones physical number pad buttons while pretending to pay attention.

TLDR: Millennials grew up during a communication technology revolution and as a result they've got some hang ups about always with you communication devices. Voice and video calls are an intrusion. For many, a ringing phone signals only parents, authority, or debt collectors.