All three expressions are correct and can be used interchangeably. The first expression, "Please call at 123-4567," indicates that the person should make a call using the number specified. The second expression, "Please call 123-4567," implies the same meaning without mentioning the word "at." Lastly, the third expression, "Please call me at 123-4567," specifically states that the person should call the speaker using the specified number.
