Based on your description of the site "large 4 building complex", I suspect that the cable run from the zone paging unit to the elevator is probably very long. The original installer may not have been able to get adequate signal strength between the 1109 and the 2900. I suspect the transformers were added (with or without the sanction of Valcom) to circumvent the signal losses in the cable.

You don't say why you are trying to find the specs on the transformer, but if my assumptions are correct, one side of the transformer would need to match the impedance of the Valcom devices (600 ohms? 45 ohms?). The other side would need to be a lower impedance. The lower impedance side of each transformer will connect to the cable pair between the buildings. The higher impedance side will connect to the Valcom devices.

The value of the lower impedance side of the transformer is not specifically critical. However, if you need to replace a transformer, you will need to replace both transformers with a matching pair.