Controlling various Arduinos with Android
If you're willing to sacrifice a cheap phone, take a tip from terrorists all over the world: the ring signal in a cell phone can be detected easily and used to power all sorts of useful detectors. 

Open up a cell phone and find the two leads that connect to the ringer. Route these to the master Arduino, and have it detect a signal on the lines. You might need to do a little investigation to find out exactly what the signal looks like, but that ought to be straightforward.