Hi Kathy -
We experienced the same problem, but were never able to pin down a root cause. This happened to 2 users out of about 50 that have call forwarding turned on for their cellphones. We were not able to find a solution - as a workaround, we advised our end users to always check the "Use the voicemail service of this external number" box as a best practice when using call forwarding.
Hope this helps!
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Cory King
IT Lead
IGS Energy
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