In a remote office, we have two 7961G. Each phone has 2 expansion modules 7914.
Once o twice in a day, phones stop to work, all buttons become red and you cannot ping the phones. After a long check phones begin to work again.
In the same office there are some 7940, 7985 and two 7961 without expansion modules: noone has this problem.
In the phone log I found some java errors with connection timeout.