Explanation:

- After two months: This phrase sets the time frame for the return of phone signals. It is grammatically correct.

- The phone signals: Refers to the signals from phones. This noun phrase is correct.

- Were back: The past tense "were" is used here. It correctly indicates that the signals returned.

- At the valley: The preposition "at" is often used for specific points, while "in" is used for enclosed areas like valleys. The correct phrase should be "in the valley."

Correct Answer: 3 - at the valley