The only 2 wires available to the Business Telephone equipment room was a single phone pair of 26 guage wire. I could not switch much current by these two 26 guage wires except power a relay coil.

Then it occured to me that if I had a polarity sensitive relay I could power up the only 2 wires with a DC voltage to turn on one relay with one polarity,  and reverse the polarity to turn on the other relay, as required for normal operation. Both relays would normally be de-energized waiting for those 2 wires to power one of them up.

The Business Telephone system provided 2 Normally Open contacts that would provide a 5 second contact closure when activated.