You can also transfer your home BT phone calls to a mobile...the advantage is that the number can be viewed so that you can decide if you wish to answer it. I used to do this when I was on dial-up and it worked well.
To transfer your BT line to any other dial *21*your new number (moby or landline)#. To turn off dial #61# I think you might have to request the service but BT have never charged me for it. If you do not answer there is no charge so you can monitor the phone call and see if it is worth answering or calling back later.