You can record using the Mitel software or by dialing into the voicemail account.
If dialing in using your phone after pressing the voicemail button and password, press 7 and then 1 to record greeting.
If using the Mitel Connect software, Click the drop down from the top of the software, select Settings… Select Call Routing, Select Available to record a greeting for that Call State and select Record
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You can have a different recording if you select out of office from the drop-down box where you see Available if your message needs to differ from the Available recording.
Mitel Quick Reference