Are you doing "call forwarding" using the set, or are you doing "call rerouting" in the system programming? If you are doing it from the set, then which call forwarding did you enable? There are a few of them.
Call Forwarding options on the set include:
Always -: all calls immediately are forwarded to the destination.
B-INT -: only when your phone is busy AND the call comes from an internal source.
B-EXT -: only when your phone is busy AND the call comes from an external source.
NA-INT -: only when there is no answer on your phone AND the call comes from an internal source.
NA-EXT -: only when there is no answer on your phone AND the call comes from an external source.
I suspect you have only programmed NA-INT. Call forwarding on the set will take precedence over call rerouting for the set. If you do not enable call forwarding from an external source, then the call rerouting already programmed will be used. In your case, the call rerouting is to voicemail.
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