Grandstream IP Audio conference Device. The GSC3505 is a 1-way public address SIP speaker that allows offices, schools, hospitals, apartments and more to build powerful public address announcement solutions that expands security and communication. This robust SIP speaker offers crystal clear HD audio functionality with a high-fidelity 8W HD speaker. The GSC3505 supports Bluetooth devices, a built-in whitelist and blacklists to easily block unwanted calls, SIP and multicast paging, and integrated dual-band WiFi.