Found the answer! It's because the calls are being diverted to the 1571 voicemail service. (even if you didn't think you had it) if you dial 1571 it will play back the messages and you can delete them. But it still won't let you use your 3505 as an answering machine. You can get it disabled by calling your service provider. I'm with Virgin but it's the same for BT.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cordless phones.