So why is "mon téléphone" not accepted when in earlier lessons it was?
It is right. When french people talk they do not say most of the time "mon téléphone portable", they prefer to say "mon portable". But it is totally right...
"Forgot"is the wrong word in this sentence.
The right word is "left," because that is where the phone is. The "forgetting" takes place "everywhere that Mary went" before the phone was remembered.