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!