The baby monitors I have experience with have two channels. The channels are labeled "A" and "B" with no indication of what frequency the two are actually using. Different models seem to use different frequencies for similarly named channels. I make this assumption because two base stations of different models configured to transmit on "A" are not both picked up by the receiving baby monitor. Two monitors of the same brand seem to cause interference if both transmit on the same channel, because the receiving monitors only pick up static.
I'd like to have a single receiving monitor for multiple transmitting monitors. I don't like having to set up multiple monitors to hear if the children are crying while I'm in the basement at night or working outside during nap time. Is there a monitor that allows me to set up multiple monitors in the children's rooms (preferably 3), configure them all to transmit on the same channel, and only requires that I have 1 monitor to receive sound? I don't need to be able to identify which child is crying (as I could when each room had it's only individual monitor/channel). I just need to be able to hear them all so that I can walk into the hallway and determine who's making noise.