If you are on an active call, place it on hold, answer another incoming call and try to transfer or blind transfer the call you just answered you get invaild number. Maybe being able to handle more than one call is asking too much.
Each button of the phone can handle 2 concurrent calls at a time. In your case, you already have 2 calls with the button, there is no call available if you tranfer or blind transfer the call which requires another call.
Hope it helps.
Best regards,
Wendy
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