I Can’t remember if this particular app has been mentioned here before but it basically allows you to use your iPhone with all calls being charged to your BT bill. This means, depending on your plan and whether you have a pay as you go mobile deal, many calls are going to be free. It’s pretty straightforward to register and even easier if you already have a login for bt.com You will provide your home phone number, your mobile number and then go through a verification process where a code is sent first to your home phone and then your mobile to complete the registration and activate the service The app will automatically import all of your iPhone contacts and it is very accessible and easy to use from there, effectively replacing the phone app when you want to make calls to numbers you get free calls for on your BT plan. https://itunes.apple.com/gb/app/bt-smarttalk/id586965407?mt=8