Plus if you do get a message whilst your on-line and log off...you cant log on again until you have taken the message. The free 1571 has caused me so much grief as we broadcast on a telephone line (different one every day) and if the owner of the venue is out and they receive a message, were snookered.
You can keep the BT answer phone. I have found that so often people forget to retrieve their messages.
A stand-alone answephone can be bought quite cheaply these days, and you do not need the PC to be left switched on. I have Phone-Tools suite that allows the PC to receive messages and faxes, but I don`t use it. I am satisfied with the the Fax to e-mail service from Trinite, and my normal answerphone.