O2 are pretty good at it as well.
You sign up for a package - "well we will charge you this for the 1st month and credit it back for Month 2" Ok - but where is it after bill 4 has come through and they have been charging at £7-50 per month above my agreed tariff.
I go in yesterday and ask for a sort-out. "Yes we can see the credits need adjusting but we are going to DD you for the current bill and then we will sort it out for month 5"
Like feck you are - that nearly £40 you owe me and more than offsets this current bill. Now assume there are some fifty thousand of us in this position, that's £2 million of funds owing that can be placed on the overnight money markets where interest rates can be 1/2% per night........that's a bucket load of cash coming to O2 at my and other's expense and it's not on. A judicious letter to the Telegraph is in my mind now....."
Back she goes, online to some invisible slave and, very reluctantly, they agree to correct the outstanding to me over this immediate bill and the next one. Very reluctantly indeed - and left me with stating as someone who has been a senior exec in the Gas business (not BG), we hadn't even dreamed up customer service of this lack of sensitivity and use of customer funds.