BT Mobile have introduced a new £12 a month Sim if  BT Broadband is used at your Home Address. Otherwise £17 a month.

Unlimited Minutes.Unlimited Texts.2 GB data. Unlimited BT Wifi.

http://www.productsandservices.bt.com/products/4g-sim-only-plans/bb.html

If you have inclusive minutes to 01/02/03 0845/0870 on your landline you can use them via your mobile.Using Smart Talk App
http://bt.custhelp.com/app/answers/detail/a_id/40735/~/how-does-the-bt-smarttalk-app-work-and-what-are-the-benefits%3F

They let you move up as many plans as you like as many times as you like. They will also let you move down 1 plan once within your contract.