With iOS 27 Apple introduced the iPhone handoff, which allows sharing the same phone number with two devices [1]. Maybe their thinking is that Duo is not necessarily your only iPhone.
[1] https://support.apple.com/en-us/127274
So you're meant to shell out $2000 for the Duo and then an additional $1200 for an 18 or something?
So you're meant to shell out $2000 for the Duo and then an additional $1200 for an 18 or something?