A teleson is a mage-made device that amplifies Mindspeech. They were common both before and during the Mage Wars. They fell out of general usage after the Cataclysm made them, and other magical devices, unreliable.
The expedition to the remains of Urtho's Tower found a prototype of an improved teleson in Urtho's remaining workshop. They used it to stay in touch with the artificers, mages and Grand Council of the Alliance in Haven during the Mage Storms. The new version only required one person with Mindspeech to hold the connection, allowing those without the Gift to make use of the device. This was demonstrated when Altra held the connection open for Karal and Natoli.