The switchboard model is going to make it a lot worse than telepathy. Consider the action flow:

• Alice wants to send a message to Bob.
• Alice uses her action to send her message to Switch (who, let's assume is paid to give this job his sole attention.)
• Switch must then spend their action to send to Bob. Bob can freely reply.
• Switch must then spend another action to relay the reply back to Alice.

Switch at any point may need a message repeated costing another action for the sender, and is more likely to need a repeat when messages are sent in stressful situations.

If Switch has other duties, the party might need to arrange ahead of time that they are going to need the service. The service also lacks efficient broadcast capabilities. Each stone needs to be activated sequentially, and Switch can reasonably only be expected to understand one person talking at once.

I think it sits closer to telegraph than a moble phone. Depending on the kind of campaign it might represent a vulnerability that can be exploited.

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 🤣😱☠️

let them buy it from a local vendor who's actually the BBEG.

then have them roll for perception every time the stones are used, crossing their wires and sending them on wild goose chases until the stones are unfucked by some magical authority or the players stop using them because they realize they're compromised.

You know, an inexperienced switchboard operator could easily make the wrong connection, especially in a stressful moment...

Next thing you know, judging from comments here, the players will give up on magic and ask for some copper cables going all the way back to their base

Sounds like fun! I would reward the engagement with the stones they want. There's room for all sorts of ways for it to work really well or really poorly. If they're consistently using it to circumvent your story beats, maybe some interesting complications or particular defenses might be in order.

Man, I wish I could play these games with people. All of these comments sound fun as hell.

The most obvious thing to do is have the NPC be replaced with a doppleganger minion of your next big bad so that the sending stones end up passing along disinfo (splitting the party, giving wrong directions, getting them to discard a magic item under the false belief it is cursed, etc)