Go to Settings at the bottom left of your menu, and then click on Phone Numbers. Click Add Number, select your country and search for a number using your desired area code. Not all area codes will be available, so it's good to have a backup. Find a number that you like and click save.
Click on the pencil icon next to your new phone number. Follow the video for detailed instructions on what each setting means. Adjust your settings to your preference and save.