A while back, my POTS line developed an open tip wire, and it occurred to me that I should still be able to get dialtone by substituting earth ground for tip. Before widespread AC power, this used to be more common as a way to do telephone and telegraph lines with just one wire instead of two. However, now that there is AC power everywhere, the disadvantages of this practice become obvious, at least in telephony.
The reason this works is due to the way an analog telephone line is connected to power at the CO. Tip is connected to talk battery return, which is bonded to earth ground at the CO while ring is connected to talk battery (-48V). Even without a wire for the tip connection, since battery return is bonded to earth at the CO, we can use the earth itself as the return path. Due to the large loop area this results in and lack of any twisting like you'd have in a proper twisted wire pair, this arrangement is highly susceptible to picking up interference. Power line noise, of course, is very prevalent.
I also have some video here explaining a bit more: https://youtu.be/-7vaD58Uj58