There are two types of analogue ports within the VoIP gateway world. FXO – for connecting phone lines to, and FXS for connecting phones and faxes directly to.
If you are going to connect to a BT socket on the wall for example you would require an FXO gateway. This would allow you to send calls from the IP network to BT and BT to the IP network.
One word of advise…. nearly all the gateways ship with a US telephone jack (RJ11) and you’ll probably need a converter to change to the BT style. These are readily available at most PC shops.
BT use Loop Start on their lines so configure the unit for that if it is an option on your particular gateway.
If you need any further advice please feel free to contact me at: email@example.com
Hope this helps.