It's good news if you have a BT Business phone line, as 1571 is free.
The BT Answer message service is provided by default. All unanswered or busy calls are redirected to a voicemail server. Note that features such as Call Waiting and Call Forward when Busy override the 1571 busy divert.
Dial 1571 to retrieve your messages. Due to technical limitations you will not receive any notification on your handset that you have messages waiting for you, so you should check the service periodically for messages.