Unlike Call Minder (for which you have to pay a fee), BT's1571 service doesn't allow you to personalise the outgoing message. So, for example, you can't record "Please try my mobile phone; the number is . . . "
1571 also records only 10 messages, which are only stored for 20 days. (If you go away on holiday, a caller might be able to leave a message or it might get deleted before you return).
Further, most answering machines allow you to access your messages from any other phone (by calling your home number and entering a PIN), so you can pick up messages left on the machine from your mobile phone. 1571 doesn't provide such a facility.
Chris