Is there any way to force a survival challenge to start?

One of the things I still have left to do is a whole bunch of survival challenges. These challenges occur when a friend of the Saints calls you on your cell and asks you to show up at a location and kill or survive waves of enemies.

However, it seems like these calls only happen randomly, and I haven't figured out any sort of pattern to them. I don't have much left to do except for these, so I'd like some way of "forcing" them to start. Is there any way to do this?

I know the full strategy now;

• You only get calls when you're on foot, so avoid entering any vehicles till you get a call
• You also can't have any notoriety at that time
• You won't get called while on a mission or activity
• Entering diversions (like vehicle surfing or hostage) cancel the challenge

The location you're in doesn't seem to matter as I've gotten calls that sent me from one side of the map to the other.

The best I've come up with is:

• You won't get a call to start a Survival challenge if you're in a vehicle.
• The calls never come when you're engaged in some other activity or plot mission.

The best thing you can do is go find someplace quiet and wait. If you've got some other challenge you could participate in, one that leaves you on foot and otherwise unencumbered by activities, you can kill 2 birds with one stone by doing that while you wait.