@Cath77 No you can't turn it off, which I'll admit seems strange, but you should be able to set the number of rings before answering on your own phone to less than the 1571 setting, so your phone picks up first.
Yes, you can turn it off. You have to select 'Cancel calling Features' and complete the form.
Thanks for that then @licquorice . I stand corrected.
Thought does occur, however, that leaving it on and setting it to more rings that your home phone may be useful if a power cut knocks out the DV.
Good point.
This no longer works. You get a page which oddly is addressed to BT staff rather than customers saying that the case should be referred to Sales Support who will ring the customer. So, I tried ringing Sales Support. At the first shot, the waiting time was 30 minutes, so and rang off and tried later. The second time I got through in under 10 minutes and was told that it could not be turned off, but that the ringing time could be extended beyond 10 rings, which would have solved my problem. I was transferred to someone in Dublin to do this (I had to wait about 15 minutes!). He told me that the ringing time could not be extended but the voicemail could be turned off, which he did after gewtting his supervisor's permission. I find that he has turned off Call Protect as well, but maybe that can be turned back on. I am gathering my strength before trying that! I can't imagine why, if it is as easy as that to do, BT make it so difficult to find out how to do it.