The background story is hard to explain (especially as I have barely followed it myself). Suffice it that — due to some repairs or renovations on their house — this family has been living outdoors in an extended backyard for a long time, variously in tents, or in this treehouse, or in the scrub with local wildlife.
(The one previous appearance of Home Free on CIDU was from the setup of this situation. We didn’t get it then, either.)
And Yay! to the cartoonist for that “Ring Ring Ring” in the first panel! I completely accepted it as a drawing device to indicate the electro-mechanical sound of a phone ringing. But no, it was the wife just saying the words “Ring Ring Ring”. Good job of surprising us with a defeated assumption!
But then tut-tut on showing the string just drooping there slackly! As of the day following the GoComics appearance of this strip, some ninety percent of the comments there are devoted to pointing out that for a “tin can telephone” to work, the string has to be pulled taut between the cans, essentially in a single straight line. (And secondary to this, the attachment of the string with a piece of tape, shown in the first panel, is a rel;ated part of why this setup wouldn’t work — to get the vibrations to transfer, the traditional setup is to punch a small hole in the base of the can, thread the string thru the hole, and tie a knot on the end so it can pull taut.)
Hard to guess whether the cartoonist just didn’t know; or knew but just didn’t think it would matter; or knew but built in an excuse, that they can quite easily hear each other by the sound carrying in the open air, and the tin-can-telephone is just their little joke!
Comics I Don't Understand 
I think Tom Toro did indeed know how those “telecans” work. In the second and third panels (when they are in use), you can see that the string is at least partially suspended. It’s only when she “hangs up” (in the fourth panel) that the string has become completely slack. The “error” is therefore confined to the first panel: her “ring-ring-ring” would never transmit to his can unless she first pulled the string taut. Whether or not that would be possible with a tape attachment is subject to argument.
I thought I remember someone “recently” mythbusting the tin can telephone — basically, they never work, no matter how taut you hold the string. It was one of those revelations as to how trusting and lazy we tend to be — anyone could try it if they half cared to, but no one seems to even half care. Just like anyone could have refuted Aristotle’s assertion about the number of teeth women have, but no one did. Sadly, I don’t remember where or when I saw this, nor do I feel like getting a couple of tin cans together and doing it for myself. (And maybe the kind of metal would have to be controlled for? Maybe it doesn’t work now with aluminum, but it did work for tin (were “tin” cans ever really made of tin?)?)
To the best of my perfunctory research, the first “tin” cans were tin-plated steel; indeed, most such cans were tin-plated steel until the development of aluminum cans.
That said, the acoustical properties should be fairly indistinguishable. If anything, aluminum cans should work better, being generally thinner than steel cans.
The principles of the acoustic telephone are well-established, so it’s not clear at all why the tin-can version shouldn’t work.
I think they probably are supposed to be slack, except when hung up. Like a home phone, the kind with an extra long cord. You don’t just sit there by the phone, you walk around until you run out of cord.
And a cord running through a hole and tied in a knot would be a very hard thing to draw, so likely he just drew tape so it wouldn’t look like the cord was just hanging there.
Nope, I don’t see the 2nd and 3rd panels as showing a configuration in which the telephone could work. The string is still slack and drooping. It’s not as though it is a hollow tube and is meant to carry vibrations of the enclosed air — in which case some slack and droop would be okay. But for the string to transmit longitudinal pull-and-relax vibrations, it cannot be slack, at all.
The last time I experimented with “telecans” was nearly a decade ago, but as I recall, the primary problem wasn’t the can, nor the attachment, but the material used for the string (the stuff I was using was probably too thick). I considered switching to thin wire, or even a long rubber band, but never actually did the experiments.
P.S. Almost all cans for food are still made of steel, because it is a lot cheaper (and more sturdy) than aluminum. Soda cans are the (major) exception, but the indented hemispherical “punt” at the bottom of an extruded soda can would be very poor at transmitting vibration. The flat surface of a steel can provides a more flexible membrane, which should provide a much better result.
Anybody here ever buy or send away for toy “space phones” (No batteries needed! Range of 20 feet!)? Very faintly recall a thin film of some kind inside the plastic that picked up the vibrations from the string.
M.A., sounds like an interesting variation, but this is the first I’m ever hearing about it!
Some setups described online use plastic cups, of the Solo variety.
An ancient (1,200 year old) telephone, similar to a tin-can telephone, was found in Peru. It is currently in the Smithsonian National Museum of the American Indian. https://www.ancient-origins.net/artifacts-ancient-technology/1200-year-old-telephone-amazing-invention-ancient-chimu-civilization-020490
I am racking my brain as something in the back of it says my dad once did a can and string setup for me or me and my sister – but I am not sure, though it is the type of thing he would do.