Assuming that you've got the basic BT 1571 service on your phone line (rather than BT Call Minder, which you have to pay for but gives you more options), you can't change how long your phone rings before it cuts in. Your phone will ring for a fixed time and then the 1571 service takes over the call. Your phone is set to cut in after a longer period of time, so 1571 gets there first.
Page 23 of your phone's manual shows you how to reduce the time before it's built-in answering machine cuts in. You need to set it to a time which is shorter than that used by the 1571 service, so that your phone's answering machine cuts in first.
Alternatively, see 'How do I remove BT Answer 1571 from my line?' here, to get rid of 1571 completely:
http://bt.custhelp.com/app/answers/detail/a_id/8374/~/bt-answer-1571-voicemail-on-your-home-phone