Source: Eberron - Rising from the Last War

Wondrous item, very rare

The key to long-distance, virtually instantaneous communication across Khorvaire is House Sivis's network of message stations. Each station contains at least one speaking stone , which is carved from a Siberys dragonshard and inscribed with arcane symbols that uniquely identify it. If you're a gnome with the Mark of Scribing, you can touch the stone and use an action to cast the Sending spell from it. The target is any other speaking stone whose location or unique sequence of symbols you know. A creature within 5 feet of the stone hears the message as if they were the target.

In a Sivis message station, a gnome is always on duty by the speaking stone, listening for messages that might come in and transcribing them for delivery to their intended recipients.