Nathan,
I am assuming that you have a Microfilter on each on your phone lines ? please confirm
There are some issues with the BeoTalk 1100 & 1200 where either the line stays open for some seconds/minutes after the call is terminated on a few calls and/or a message is left, but there is no NEWS number, so the user does not press the news button and so often does not get messages. Both these issues are intermittent.
Does your problem happen on every call ?