Not able to add SMS protect

Hi Blizzard,

I really find it odd that I’m able to add the same mobile authenticator with the same serial number to multiple accounts but I’m not able to add the same phone number for SMS protect. Any reason why this is the case? I have strong passwords & authenticators for my accounts and I also want to setup SMS protect on this account but I’m unable to because my phone number is already in use by my main account.