We have a MM1.1 for Exchange connected via PRI (QSIG) and have started experiencing intermittent dropped calls when a caller hits 0 out of a mailbox or auto attendant and the call covers back to the "0" out destinations voice mail. It will ring the "0" out extension but as it goes to coverage you hear a DTMF tone and the call drops. Happens about 50% of the time. Does not appear to be specific to any ports/PRI trunks.