Ken,
1: the most delay you can get from the TA824 is 15 seconds or about 3 rings.
You would have to use the "delayed ringing"
You could get another 15 seconds out of the TVA but then any time you call the voice mail to retreve your messages you'll still have a 3 ring delay.
2: check the message timer for that mailbox.
3: make sure that "all calls transfer to mailbox" is NO. There are some other things to check as well for the MWL but I'd start there.
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MrG