Right; if you want to use your POTS line with your OBi, you need to buy a 212.
However, I suggest that it might actually be cheaper to ditch the land line entirely, port that number into either Google Voice or some other Internet Telephone Service Provider (ITSP) and get a cellular communicator from your alarm monitoring company. It's worth a cost-comparison. They typically use either AT&T Wireless or Verizon Wireless, so, if you have coverage, it's actually a more secure method, since nobody can cut a phone line or kill the power to disable it.