My decade+ old Hotmail account, plus two more newer ones, began prompting me for a # "for security" back around 2014. After a couple weeks of "not right now" al

My decade+ old Hotmail account, plus two more newer ones, began prompting me for a # "for security" back around 2014. After a couple weeks of "not right now" all three of them locked me out simultaneously. Yahoo still asks for a # to this day(AFAIK... stopped using it after Oauth prompts appeared). Security IS one benefit, but it does not seem to be the most heavily weighted reason. Most don't change phone #s often, if ever. Seems like a super data tracking metric.