Unlimited Calls and Text for £20.00 a month. 30 day rolling contract Sim Only Value Max @ Utility Warehouse
The UK's cheapest mobile tariffs guaranteed.
They have 3 price plans and one of them is Value Max which gives you unlimited cross network minutes and text. There is no fair usage policy. The network runs under T-Mobile and Orange network. That is the cheapest unlimited SIM I've seen so far.