A PRI card is a hardware device used in telecommunication systems to facilitate the connection of a private branch exchange (PBX) or telephony server to a PSTN or an ISDN. A PRI Card works with Interfacing with ISDN, Multiplexing, Applications, & Protocol Management. Businesses use PRI cards for setting up voice calls, fax communications, and data transfer in PBX systems, call centers, or unified communication setups. The card manages signaling protocols such as Q.931, ensuring smooth communication between the PBX and the network.