If you're not using the built-in ESVM for voicemail, you can upload WAV audio files into the OGM slots using the software. WAV must be 8kHz 16bit mono. Set SVM to OUS, then disable the ESVM. Then use Utility > Message Transfer from PC to PBX. Tell it what slot/OGM to upload into (this can be very slow, be prepared for up to 15mins). Put SVM back INS, and assign your OGM number to the DISA number.