There are too many variables to know what you are missing, based on your post.
For example, security alarms generally use a special type of modular phone jack, called an "exclusion jack". It has extra pins and wiring that loops through the alarm panel. The intent is that normally, phones plugged into the house wiring can get a dial tone and make/receive calls. When the alarm is triggered, the panel cuts off (excludes) the house wiring and seizes the line to call out without a burglar picking up the phone to interfere.
You need to know how those jacks are wired, and how to correctly connect the phone cord from the OBi to the jack.
Aside from that, there's no guarantee that it will work at all, despite Obihai's claim. Using a telephone line connection to the central station is pretty much obsolete. If you are an ADT customer, you should have an ADT Pulse gateway, which handles central station communications over an internet (Ethernet) connection, with a 3G/4G cellular communicator as backup.