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.