How Do I Call in a Bugling Bull from a Separate Unit?

[Bowhunt.Elk]

I hunt right on the edge of two units. One is OTC and the other is draw only. Now I usually have a tag for the OTC unit and I constantly hear bulls down the ridge in the adjacent unit. My question is what tips and tricks can I use to try and call in those bulls?

 

[hank4elk]

With some  luck,
I hunt a unit with 2 well known ones meeting ,all at the same place.
Have only called a bull across the hwy once and he came and got his girls back.Mid day.
I have called cows & spikes in.
I'm usually trying to get them before they go to other unit in morn. Close enough that I keep trying.