Not that trivial!
"Customer details are in the phone book, and most people will tell you their contract renewal date if you call them up and ask (as cold callers are wont to do)."
Jeez, this might not be a hanging offence but Bill you really are going in the wrong direction with this one. The Register normally takes a decent enough stance on data theft and the need for a greater powers for the ICO.
Which phone book contains customer details? Do you mean the BT one that is now approaching the size of a magazine (if you take out the business listings) as there are hardly any customers who wish to be included in it as they don't want random marketing calls?
How on earth do you feel justified in writing the comment that people will give up their contract renewal date if asked by a cold caller? Where are your sources to back that up? The only empirical evidence I presume you have is that you, yourself, are willing to divulge that information when asked?
The cold callers in this case can use this information to deceive people into thinking they are calling from T-mobile. For example "Hello Mr Johnston, as you are a valuable customer I wish to offer you a great deal on the renewal of your contract next month..."
This goes beyond the irritation of just receiving unsolicited calls.
The other point is that the money that the staff were suposed to be making from these sales is far greater than the maximum fine of £5000 and so there is no disincentive to doing this.
It will still be up to a court to decide the penalty but I can see some cases where a jail term would be warranted and certainly maximum fines that would allow recovery of all money made and more.
Are you really happy that any of your information could be available for sale to anyone else with little repercussions? Credit applications, bank details, shop purchases, travel plans...Really?