System type is very important.
Type of Voicemail? (Embedded or otherwise)
Also how is *<ext> programmed
- Speedial?
- Phantom DN Forwarded to VM?
- Phantom DN forwarded to speedial?
- Any of 100 other scenarios?
Just guessing here, but your symptom reminds me of an issue with an SX200 with a phantom DN forwarded to a speeddial that terminates to an inband integrated voicemail.
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Occam's Razor - All things being equal, the simplest solution is the right one.