Hello Amit,
As of right now, we do not have a way to natively customize the subject and both of the notification emails. I did see this idea from the Product Ideas Lab that is asking for something similar. You may want to vote and comment on that idea.
One other thing that you could do would be to use our APIs to grab new voicemail messages and send your own customized notifications.
GET /api/v2/voicemail/messages
GET /api/v2/voicemail/messages/{messageId}
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Jason Kleitz
Online Community Manager/Moderator
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