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the right answer to your question.
I find your question very interesting.
What is the model of the mobile phone in question?
And what is the maker and model of the ATT phone?
Are both phones (the first of which I recognize as a Panasonic cordless phone, the other of which is a classic AT&T phone of the older sort, that works of a landline)... are both phones on the same phone line, through a splitter?