Intermittent problem where Doorphone does not ring phones. Replaced Doorphone, Doorphone card, and Cabling from KSU to doorphone. 816-3 Could problem be from a station or from a phone? Or from the KSU itself? Tough to figure out. Want to replace KSU next, but am afraid if I do and problem still happens, then what. This happens sporadically, once a mos or every few weeks. Stays broken, then starts working again.