Inverted phase communicators are devices used by the Tollan for long-range communication.
The communication device does not work by warping or bending space and does not require a Stargate. Its use was cryptically explained by mentioning how the two ends of a branch are not so distant when brought together. The actual mechanics of the device were never explained beyond this. (SG1: "Enigma")
After SG-1 rescued several Tollan from their original homeworld, which was being destroyed by severe volcanic eruption, the survivors were subsequently detained on Earth. They were forced by the NID to provide information regarding their advanced technology, which they refused to do. Realising that they would never be safe on Earth, SG-1 violated their orders and helped the Tollan contact the Nox using one of these devices. (SG1: "Enigma")
The Tollan provided their allies the Tok'ra with one of these devices, who in turn gave it to Stargate Command, allowing the Tau'ri to contact the Tok'ra at any time. In exchange, the SGC gave the Tok'ra a GDO. (SG1: "Serpent's Song")