If you're unable to register your phone number to your X account, it may be due to one of the following reasons:
If the same phone number is registered in more than 10 accounts
If the same phone number is registered to an account that’s been deleted in the past 30 days
If the same phone number is registered in a suspended account
Read more on our Phone Number FAQ. Note: If you've previously blocked SMS messages from a 40404 number, we recommend that you unblock or try an alternate phone number from a different carrier.