[exratione]

Many phone companies will allow you to (a) add an annotation to your account to declare the number you are using should never be ported to another company, and (b) add a password to the account that you will have to provide to customer service representatives when making changes. This helps to minimize the chance that an attacker can use social engineering to redirect your number to a system under his or her control. If these are not options for your phone company, find a better phone company.

Even given that, since it relies upon human choice and behavior, and does nothing versus attackers with assets within the phone company, it seems a bad idea to have 2FA via SMS.