I say, let them run with their idea. Sounds fun and already full of complications 🤣

For instance, you could give them a few go-rounds with this switchboard mechanism to underline its flaws, and perhaps have them encounter a theoretical mage fixated on pushing the boundaries of traditional spellcasting who says s/he might be able to create a bridge between sending stones and replace this human switchboard.

Of course, that quietly lays a foundation for all sorts of DM shenanigans:

1.) The mage is secretly nefarious and listening in on their conversations, or

2.) The bridge is somehow causing glitches in nearby messaging magic of all kinds, leading to a local investigation of this arcane static, or

3.) The mage's enchanting of the bridge went awry in some miniscule way. Completely undetected, the continued use of the jerry-rigged Weave is exacerbating the issue leading to increasingly negative effects — eg.

3.a.) Users begin hearing their distant companions as if they're right next to them, and it grows to include sleeping hours eventually preventing them from getting rest [Exhaustion is a bitch ] until they leave their stone a certain distance away.

3.b.) Something indescribable lurks in "the spaces between" that these bridged messages travel through from stone to stone, and it begins to take notice of the sudden noise/static in its surreal domain/hunting ground/oubliette/etc. — perhaps even feeding on the stones' bearers, or moving to cross over into t the e PCs' world via this irresponsible breach of ancient arcane protocol...

4.) Or, all of the above 🤣😱☠️