Here's a follow-up post for anybody else who wishes to make up their own little "Bubble-Gum phone" as shown in the NCRC communications material.
Firstly, know that a BG phone is merely a small piezoelectric element which can work as very basic sound-powered "phone" in a pinch. They can be useful as listening devices and for testing the line over a half mile or so range (if you're lucky). But they certainly don't have the range and clarity of the proper field telephones often used in cave rescue, so I suggest you don't rely on them as an effective substitute.
To build one for yourself, you'll need a couple of parts from Radio Shack. The
piezo itself and some
wires (preferably with alligator clips on the ends).
Here's the way I've done it. Carefully remove the rear of the piezo element case with a knife or tiny screwdriver. Unsolder the flimsy wires from the element itself, and solder on a couple of wires with alligator clips on the ends. I buy the jumpers in the link above, and cut the wire in the middle. Presto - 2 leads with clips on the end. Cut a small slot in the rear cover for the wires, and glue it back in place. Add a little glue around where the wires enter the case to help secure them and prevent water/mud getting in.
I've heard mounting the piezo in an original bubble gum tin or similar (maybe an Altoids tin) improves the acoustics a little. Or you can create a more compact version like I did, by finding an old 35mm film canister to stuff it in. If you do it that way, take a short length of very thin cord and tie a loop onto the piezo brackets. This allows you to hook the piezo over your ear and listen without holding it.
And that's basically all there is to it. See the photo in my previous post for a visual.