BT are upgrading all the old analogue telephone lines to new BT Digital Voice. The Digital Voice service includes a free voicemail service. The video shows how to setup the voicemail service.
Thank you so much - when I tried the BT website option you described, BT's website had an error message which said it couldn't access the service - how absolutely typical! So I used the 1571 option on my handset which you described. Now at last I can bypass their Voicemail jumping in before my handset's own answerphone message. Brilliant - I am very grateful to you.