If you allow this (even if you can) you will end up with people moving old voice mails that they cannot be bothered to delete, that have been out there for years. When we moved over, we kept the old voice mail turned up on an alternate number and allowed them to dial in to listen to their old messages for a period of two weeks, 2 weeks - so we would not miss anyone that may have been on vacation the first week. This worked out well, if they needed information, they could write down the information, or record it with their own hand held device it they wished. We only got one complaint - from a person with over 150 messages in his mailbox, dating back several years in some cases!