It almost sounds like the comapny was planning on you taking the extension, when they called, this is not correct. When you reach the end of your FDP and the comapny asks for you to extend you may tell them yes or no, but they are NOT supposed to factor in the extension before the flight. The purpose of the extension is for operational recovery, not planning. Keep in mind if your extension is greater than 30 min up to two hours, you are required to have 30 hours rest before you can extend again within 168 hours.