Yes, SkykingOH is correct, you have to patch asterisk, which does mean if you’re running a distro you can’t (easily) update.
Plus then there’s a few other settings to include.
If you need more help let me know.
I’ve found the 8.5.4 firmware works best on the 7961 and 7970/7971.
Once you get it working you can pick up the busy lamp field by adding this to your extensions_custom.conf file:
[from-internal-custom]
include => app-blfpickup
[app-blfpickup]
include => app-pickup-custom
exten => _[x]-cisco-serviceuri-blfpickup-.,1,Macro(user-callerid,)
exten => _[x]-cisco-serviceuri-blfpickup-.,n,Set(PICKUP_EXTEN=${AMPUSER})
exten => _[x]-cisco-serviceuri-blfpickup-.,n,Pickup(${EXTEN:29}&${EXTEN:29}@PICKUPMARK)
exten => _[x]-cisco-serviceuri-blfpickup-.,n,Hangup
Of course the extensions need to be tagged in the correct pickup groups.