That's just a firewall.
Yes you can use the pfsense as PART of your connections to BT. You will still need a Modem to connect to BT.
So it would look like
BT Faceplate >>>>> Your Modem >>>>>>> Pfsense >>>>>>> You local network.
Remember that the Modem has to connect to the BTcircuit ( I have used the Netgear DM200 before)
The modem will "speak" PPPoE to BT and conventionally it will have an ethernet port to connect to the Pfsense.
Regards
S