Show open pots lines on phone

I am new to trixbox and I am going to be installing a system that has 4 analog lines. At home I am using Grandstream GXP2000 phones here is my worry. If all 4 lines are used up is there a way to show a new caller on a Grandstream phone that all lines are used up. I just don’t want to get the employees picking up the phone and getting a dial tone only to find out all 4 lines are busy.

Any help with this would be awsome