[glatisaint]

They want a phone number to make sure you're not a spammer taking over the account.

[codedokode]

Then the message about someone else trying to get access to an account would be a lie. Also it is unclear how a phone number can prove that I am not a spammer. I think that just looking at a number of emails sent would be a better metric.

[userbinator]

A spammer probably has many throwaway phone numbers to use, since they are likely to care much less about companies harvesting data anyway.