BLF on FXO lines


I have 4 analog lines going in FXO PCI card. and each line has it’s own dial plan to dial. For example, in order to call using line1 the dial plan should start with 9. line2 => 8, line3 => 7 etc…

All I need to do now is to give the users the ability to know if one of those lines is already in use by using BLF. I don’t know how to do it or how to configure it. Please any hints?