Question : In between Asterisk BLF

I have two Asterisk servers connected together with iax2. All the extensions at both servers are connected by sip. All phones are Grandstream GXP2020. The BLF function works fine inside each server, i.e. if extension 100 calls extension 101 on the same server, the leds on the phone change accordingly to show the call status (in call (solid red), ringing (flashing red), idle (green)).  But if extension 100 will call extension 200 (connected to the other asterisk server), the led will remain green as was prior to call. Please advise on how to carry the Subscribe and Notify info through the channel SIP->IAX2->SIP. Please see bellow an extract of one extension.conf regarding BLF. The other server has one that is alike, just other extensions.

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[BLF_Group_1] exten => 603,hint,SIP/603 exten => 605,hint,SIP/605 exten => 500,hint,iax2/asei/500 exten => 501,hint,iax2/asei/501 exten => 502,hint,iax2/asei/502 exten => 503,hint,iax2/asei/503 exten => 504,hint,iax2/asei/504

Answer : In between Asterisk BLF

If you are using Asterisk 1.6.1 or higher, you will have distributed device states available. If you're less than that, you would need to use a SIP trunk to connect the two boxes.

http://snapvoip.blogspot.com/2008/06/distributed-presence-for-asterisk-in.html

You could, technically, use some centralized database techniques, but that seems like the long way around the barn.