I am not ATT's support but if I was to try to solve this with an extender, this is the system I'd try.
I have an AT&T (ZTE WF720) wireless home phone system. We are using AT&T (Advanced American Telephones model: Cl81301) cordless phones. We have a detached work shop that is approx. 150 ft. from the house. If you stand at a window facing the house you can use the phone but if you move away from the window you lose the signal. Is there a range extender that can be used on this system or a booster that can be used to cover the complete shop.

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