The GSM-PRI Gateway allows 30 GSM SIM cards to be connected to an existing PBX using a PRI line. When the PBX extensions make outbound calls, the gateway selects one of the SIM cards to place the call. Incoming calls to SIM cards are sent to the designated PBX extension. Using a single gateway avoids issues with multiple individual phones like space, power supplies, and service issues. The gateway provides benefits like a single point of management, applications like auto-attendant and call billing, and cost savings over individual phones.