Which of the following expansion cards would be necessary to increase fault tolerance in a VoIP server?

A second modem is generally irrelevant for a modern VoIP server, which relies on high-speed network (Ethernet) connectivity rather than dial-up modems for communication.

A second CPU increases processing power and general system performance but does not directly add fault tolerance to the VoIP-specific communication or interface capabilities of the server itself.

A second HBA (Host Bus Adapter) provides redundancy for storage connectivity, increasing fault tolerance for data access, but it does not enhance the VoIP server's ability to handle calls or connect to telephony infrastructure.

A second PBX card (Private Branch Exchange card), often referred to as a telephony or voice card, provides redundant physical interfaces for the VoIP server to connect to external telephony lines or networks. This ensures that if one card fails, the server can continue to process and route calls, thereby increasing fault tolerance for its primary function.