I have multiple backups here myself - my regular cable, a dialup internet (through a different company, with a REAL land line, not cable modem), and then two cell phones with data transfer capabilities on them along with an AirCard for my notebook. If things REALLY went crazy, I have a satellite phone, but that's 9600bps - handy when finishing on the boat in the ocean a few years ago, but I rarely use it - but still a necessary business expense.
Dont' forget that the phone connection isn't the only thing that can fail. UPS Backup power supply is constantly plugged in (and tested each month), a gas generator (should someone take out an electric pole/transformer somewhere and solar power (not much help in a storm, but I run deep cycle marine bateries on them, so plenty of power avaialbe for 24-36 hours without batting an eye.
Running an internet business means NOT having a single point of failure. Just like you make backups for your websites, you should have backup systems in place should various events happen.
Maybe I go overboard with my backup protection - but when 100% of my business depends on me being online and reachable, paying the extra few hundred dollars each month is a necessary business expense.