How to switch positive/negative voltage to positive voltage
I think you may actually need a bridge rectifier - it converts any polarity DC (or cyclic ac) into the same polarity: -



You can buy them as one lump with 4 leads or make them from individual diodes - make sure the voltage rating on the diodes is high though - I'd choose at least 1000V rated diodes because telephone lines do have a tendency to get induced voltages from nearby lightning strikes.