You can force users be prompted to change their voicemail password and record their name at first login by adding the following code in /etc/asterisk/vm_general.inc:
forcename=yes
forcegreetings=yes
In this scenario the voicemail's password must be the same as the extension number. And, if the user ever changes the voicemail password back to the extension number, the user will receive the intro message again, but will only prompt the user to change the extension's password again.
Alternatively you can use Dickson's custom code found here
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