Custom HINT creation for ring group


I’m trying to build a BLF hint for ring groups. The expected behaviour I need is that the light will blink when the ring group is being called. This can be easily achieved by using:
Ring group number 555 and extensions 100 and 101 associated with this ring group.

exten => 555, hint SIP/100&SIP/101

Whenever 100, 101 or 555 is dialed, blf will blink. I would prefer it to blink only when 555 is dialed, but, this behaviour would be ok to.

Problem is that, supose a call to 555 is done, and 100 picks up, blf stays red; and I would like it to be red only and only if ALL extensions are busy!

Can this be achieved???