There is a 4 digit code that goes into a zetron keypad attached to a XTL 2500 radio, this sends out a tone on the selected freq activating the pagers.
That's more than a little helpful. IIRC, depending on the exact model of the Zetron encoder, the four digit code that you enter is converted to a tone pair and it's not the same everywhere. You have to know how the Zetron at your dispatch is programmed to know the tone pair. Once you know the tone pair, you can set up your minitor to decode those tones.
For example, we had a very large Zetron in our dispatch center. The radio guys would program a four digit code to be whatever tones were needed for a specific department. So what tones were generated by our Zetron for any given 4 digit code was unique to our center.
You need to talk to someone knowledgeable at your dispatch to get the tone info.
How you are going to get the backup base to generate the tones is a different issue, or does it already have an encoder, too?