How ERS knows the trunk id?

G6.1+AVAYA G3r

We use both ISDN and CO trunk. For CO trunk I can use OtherDN to know which trunk the incoming call is using. The value of OtherDN turns out to be something like t5.6, of which 5 means the trunk group 5 and 6 means the 6th channel. But for ISDN trunk, I can’t find correct function to get the trunk id in IRDesigner. Could anyone help me to find out what should I do? Thx