I have a MerMsg with selector codes which work correctly on most calls. Occasionally a caller will punch a code and hear ringing- the called party answers and the caller hears the ringing stop but both ends get dead air. We tested on-site and sometimes made 50 calls or more w/o a failure. Tried removing 1 port at a time from the IntegVmi calling group but still get failures at about the same rate. Power cycled system but no change. Replace VM??