Sentry is a SIP back-to-back user agent that connects a PBX such as Asterisk, FreePBX, or FreeSWITCH to mobile applications. When a call comes in, it wakes the mobile client with a push notification while keeping the signaling between the PBX and the phone transparent. Each call leg is managed independently, which keeps the gateway compatible with strict PBX setups.
It is built in Go with PostgreSQL and SQLC, supports SIP over UDP, TCP, and TLS, and handles push delivery through Firebase and APNs. It also monitors shared upstream gateways, repairs registration state after outages, and scales its registration rate based on how the PBX responds. It is designed to handle 100,000 or more users.