Base Transceiver Station [BTS]

I think you have misread the specs

How can it run on minus 48 volts ? That would be 48 volts below nothing, so really really nothing

Please supply name of base unit and or serial number

Short Answer: Cathodic protection, but nothing to do with the BTS.

Long Answer: Most telco equipment, including wireless, runs on -48VDC. The twisted pair of wire that arrives at your house from the Telco's Central Office (CO) carries a DC voltage, a AC signal (your voice), and another AC signal for your ringer. Your house phone is connected to a ground system to protect you from lightning strikes, etc, and this is connected to a rod driven into the dirt in your backyard, or wherever the telco wire meets your house.

Because there is a current that runs through the wire and also to the ground rod, the ground rod would rot away ... unless the current was reversed. In this case, the ground rods in every backyard remains intact.

Back to your BTS ... all power systems in the CO are based on -48VDC, so when wireless was introduced, they kept the standard so that one could share power plants, etc.