Check your bill, it may save you money!
 

After a cockup with VM billing me for something I had registered to have for free (Tickles), I questioned the bill and was told they would put Talk 08 £2 on my bill, but give me a rolling discount of £2 each month for the duration of my contract.

All was well for 8 months, but yesterday I noticed the rolling discount had vanished, with 4 months to go.

I called today and was given a one-off discount of £8 to cover the rest of the contract year. No explanation, no quibbling.

Check your bills people!