FXO (Foreign Exchange Office) and FXS (Foreign Exchange Subscriber or Foreign Exchange Station) are two types of analog VoIP gateways that connect between two and 24 legacy phones. An FXS encrypts a phone for VoIP, while an FXO connects an entire line to POTS. FXS connects one phone (per port) at a time, and FXO works in partnership with an IP PBX for the purpose of making phone calls to the PSTN. Digital VoIP gateways (Ucaas Providers) encode digital devices such as what an ISDN or a primary rate interface (PRI) can do.