The issue might be how the dishes are set up. Before, Bell used to utilize the sw44 switch and now they use another method. The older method required two cables to be connected to the receiver, whereas the newer method only requires one cable to be connected. My guess is that you're set up the new way and your parents are set up the old way, which is why you are having this problem.
I'm not well versed in Bell's systems so I may be somewhat wrong or another thing is causing this issue.