See if your line works by dialing Intercom 805. That will connect you to the new line directly.
If you have a 6 button set (2 intercom keys and 4 line keys) you would have to put the system in hybrid mode and pool the lines to access the new line with a button.
If you have 12 button or larger sets, assign the new line to each of the extensions you want it on by using program #301. Once assigned, it will automatically be programmed to ring on the extension. You may need to remove that on some sets.