I just tested it again to be sure. I took my trusty western electric 2500D and plugged it into that box. I called into it with my cell and answered. (Puts out nice ring voltage-Loud and crisp) Anyway, I hung up the cell while lightly blowing into the transmitter of the 2500 (listening to the sidetone). Forget about listening to the sidetone die if the loop opened, it went INSTANTLY back to dial tone. Maybe I can win the powerball and just buy the local CO and not have to worry about this crap.