Had one come in one time calling with a friend. It came down a cow trail toward us moving rather leisurely. My partner was sitting with his legs in front of him across the trail with his AR across his lap pointing up the trail. As the opossum came closer I could see my buddy slowly moving the AR keeping it on the critter. At about 3 feet he flicked a stick at the possum and it turned and left just like it had come in. Only time I have ever seen one while out calling.
Have actually had two possums come wondering in when I was using wacky woodpecker, also cant say it was coming to the call but last year killed an armadillo while coon calling.