Not sure about eastern MT since i havent hunted birds there however i have hunted the black hills of SD a lot. They call exactly like a eastern bird or a rio. I have found early in the season they are not real responsive to calls but if you go a lil later they get better at coming to the calls.
I hunted them by finding open areas in the bottom of high basins getting in real early and waiting for them to come off the roost. They would roost about half way up the hillsides (usually on a north facing slope i found). My advise i go early if you can and do some scouting when they start breeding habits and are all flocked up together then if you can go back a few weeks later (if you werent successful ) on your first trip and start at the same place you found a big flock they will have broken down into smaller groups and shoud be easier to call.
I havent hunted it but if it was me and i was gonna head to that part of the country to hunt id start in the Custer national forest and some of the Blm and state land that may have river or creek systems moving through it.
Here is a couple i took in SD.
Good luck let me know if i can do anything else to help