BLF for intra-company extensions

Hello,

Here's my setup; I have 3 branches all with trixbox servers and extensions running off them, they are all linked together using IAX trunks and these work fine. The branches are connected with hardware VPN links.

I have a SPA962 with sidecart, the BLF works for extensions within that office, however when put the other trixbox's IP address in the BLF string the LED glows orange.

My question is simple, how can i ... if possible, get the BLF to showup for extensions in other branches?

Thanks

Jimmy