Check if DAHDi extension is on the phone

I have a freePBX system with a DAHDi card installed, using 4 analog phones as internal extensions. My question is if it is possible to check if any extension is currently busy? If I buy an IP SIP phone for an operator, can be displayed when an extension is busy, maybe with a flashing light or something?

Thank you very much in advance,

You can program BLFs for dahdi channels, see here: