You can probably figure out most people's default phone number by googling their name and going to one of those crappy white pages sites. Then try to login/do a

You can probably figure out most people's default phone number by googling their name and going to one of those crappy white pages sites. Then try to login/do a password reset to their services and compare the masked phone number to the ones on the site. No advanced hacking or special channels required. The trouble with compartmentalizing with a separate phone is that the company with your money probably also sold your secret phone number (that they only required for security) so it will probably also show up on that whitepages site.