The Answer: When BT saves the message direct to your answer phone without actually calling you.
Apparently BT has been caught making unsolicited marketing calls to the users of their 1571 voicemail service. Rather than actually phoning their customers though they have been simply adding the message to the answer phone. Why is this interesting? Well it turns out that as they didn't actually make a call they don't have to respect the Telephone Preference Service do not call list. Now the call in question was on behalf of a charity but this is a slippery slope. If BT get away with it this time what is to stop them accepting money from any advertising company to do the same in the future?
Of course if BT can do this then it sets a very dangerous precedent. The next step will be e-mail providers taking money to insert spam messages directly into your inbox -- if the marketeer didn't actually e-mail you then they can't be targeted by any of the anti-spam legislation that exists in numerous countries.