The STS has supervision. I ran into trouble the other day when one line wouldn't drop but others would. I found Telco wasn't sending me enough of a polarity reversal. The voltage would drop to about half a volt but not go all the way to 0 volts. The duration was OK about .5 seconds. I explained it to the Verizon Tech, he had never heard of it before but he fixed it and it works fine now.