Plo in the telephony industry usually stands for Plant Lock Out this usually occurs when there is a cableing short or a short on the equipment from the telecom provider. It also occurs when a telephone does not have a cut off on disconect feature either in the dms programing from the telecom or in the end user key system. In your case I am going to go with the water getting into the plant lines creating a short causing the lines to go into a PLO state.