I hate to use the cliché, but: weakest link. SMS and dial-back systems rely on the security of the telco, who are dis-incented to secure their users' accounts.

I hate to use the cliché, but: weakest link. SMS and dial-back systems rely on the security of the telco, who are dis-incented to secure their users' accounts. These systems do not use encryption! Of course they are going to get owned.