F&F Mobile is an old legacy plan no longer sold, it was an addon where you get a discounted rate calling mobile numbers, from a time where calling plans only included landline numbers.
Login at www.bt.com/MyBT and see what deals are online or give BT a call on 0330 1234 150, the Loyalty team can do an account review, the latest calling plans include inclusive calls to UK landlines and UK Mobiles, if you're using your landline still you'll most likely save money by renewing to the latest packages.
Also look at updating your marketing consents, BT would of contacted you a while back to switch your plan, to do so requires consent though.
