Mitel to the rescue.
press the voicemail button
press *
enter mailbox 190
enter xxx#
as soon as she says the voicemail is XX% full,
press 9, this is a hidden unannounced option
press 3 for recording options
press 1 to make a new recording
enter recording 003
listen to the greeting in full, at the end press 3 to erase and re-record
press # to save changes
press # to save