Help me understand BT Broadband phone lines!

[KAZ]

We have BT Broadband option 1 which gives you free evening and weekend calls. You do this by plugging a phone to the PC, and BT gives you a special new telephone number. My Question is...How exactly does this work? all the phones in the house will ring regardless of If you ring our normal telephone number or the new number? Is the line the same? If you use the phone plugged to the PC for calls at other times what is the tariff? is is different to the exsisting phone line?