In general, it's expected that you should be able to update your own phone number in your iCloud account.
There is a lot of disappointment expressed in the comments here but we need level-headed solutions, not just rage against things that are actually useful in 99.9999……% situations.
At this point I will just stop commenting on this post as it seems like either Apple already fixed this or some of the most critical information has been omitted by the author. So we are just guessing and raging for no reason.
Unfortunately I don't have an iPhone to check, but another comment [1] suggests that this may happen if you physically change SIMs. My brother said they didn't ask for his iCloud password, which makes sense: if they had the password then they wouldn't have needed the phishing step afterwards.
To add some context, Argentina has a history of robbers asking people to do complex tasks at gun point, like withdrawing money, and other things during “secuestro express”.
Lots of people keep talking about password codes, etc. You should be able to hand over your phone to an attacker and walk away knowing you and your data is safe.
At least don't use biometrics, they will cut off your thumb AND steal your iphone.
Also, trivia for iPhone users: my brother used to have Face ID set up, but he disabled it because he couldn't figure out how to set up a second face and it was annoying when he needed to share the phone with his wife. So don't do that!