Don't know if they are still available, but, there used to be a T-adapter that had an RJ-31X and an RJ-11 side-by-side, and of course the male plug. They used to be used for "series dialers". Your installer could terminate the line on a jack, plug in the adapter, plug in the alarm and a line cord to the phone system.
I wouldn't want MY alarm connected that wy, though....
