If memory serves me correctly, I think x9999 is IP trunks (IP networking or SIP). The "Private" calls to the voicemail, are they coming from outside the system or from another Mitel phone? If it is from outside (PSTN/SIP), then there is no way to unmask these calls, if it is coming from an IP trunk, then figure out which element and adjust the COS of the endpoint so that it is not masking it's number.