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Hi there and welcome
this is done via the call forwarding for busy / no answer
To disable it
Lift handset or press ON/OFF button.Either press the FWD button, dial  or , or use the Soft Key’s CLR-FWD option. A confirmation tone sounds and the FWD LED extinguishes.
Replace handset; ON/OFF button extinguishes automatically, if used instead of handset.
It will then stop forwarding the calls
You can see this on the manual page 2-29 available here
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ok, this is the only method I know for disabling it and I am not sure why it is not working for you
I will opt out for another expert that may have new ideas.
You do not need to do anything, you will be notified when a new expert takes over