Need help for Asterisk-GUI call handling

I have setup a new Asterisk-GUI PBX 1.4.36 with PRI trunk. I have configured the incoming & outgoing call rules as well as DID’s from the service provider. We have three business premises (example Business A, Business B & Business C). We were using Allworx 10x analog PBX before with 9112 IP phones. On these phones we have assigned lines to each flash button. The phone have 12 flash buttons. We have assigned top 2 buttons for Business A phone lines, next two buttons for Business B & next two for Business C.
Operator can easily find from the flash buttons which business line is ringing for incoming call and can greet the caller with the relevant business name. How can I find which business line is called in Asterisk? We are using Nortel 1535 SIP hard phones. Please help. Your support is appreciated.