Yes, you can!
If you would like to use a different telephone number for a particular advert to the one on your account you can do this in Step 3 of the Place an ad process.
Near the bottom of the page you will see the option Ad-specific phone number - if you put an alternative number in here, this will be the telephone number used in your advert.
If you are using the Friday-Ad App, you can specify your preferred contact number at the bottom of the Place an ad screen. Simply turn on Contactable by phone (move the toggle to the right) and enter your chosen phone number.