The Yeastar SO Module is a telecommunications interface unit with 1 FXO port and 1 FXS port, designed for integration into Yeastar MyPBX series and S-Series PBX systems. This module allows connection to traditional analog telephone lines via the FXO port as well as to analog telephone devices through the FXS port, providing flexible communication options. A key feature is the PSTN fallback capability, which maintains call functionality during system power outages by automatically routing calls through the public telephone network. The module is built with industrial-grade components, ensuring reliable performance in business environments. Its dimensions are approximately 100 mm x 60 mm x 25 mm, with a weight of 85 g. The compact design facilitates easy installation in standard PBX enclosures and supports standard telecommunications protocols for seamless interoperability with existing infrastructure. This module serves as an expansion solution for businesses that require additional analog line connections alongside digital VoIP capabilities, offering operational continuity through its failover mechanism.