I too have seen this exact problem before. You might want to ask the customer if any of their people are getting static shocks when they answer the phone. After spending countless days of frustration chasing a similar ghost, I was fortunate enough to make the shocking observation that every time the receptionist answered the phone she got a static blast. As a result I was also fortunate enough to watch the phones start to wig out just before the system would reset. The root of all evil turned out to be the results of static shocks caused by the low winter humidity, her leather soled shoes, & the offices nylon carpets. The phone would ring, she would shuffle across the room to answer it, & ZAP!!! As she wasn't too keen on the idea of giving up the clothing we had to settle on having the carpet treated with anti-static spray. Hope it helps you