Traditional PBX (private branch Exchange) provides internal call routing within a single organization, plus limited external calling, while a class 5 switch provides public telephony services directly to residential and business subscribers.

Traditional PBX (Private Branch Exchange)

• A physical on-premises telephone system for a single organization.
• Connects internal phones and routes calls to the public telephone network via trunk lines.
• Hardware-based (though modern PBXs can be IP-enabled).
• Mostly for private, internal communications with external connectivity.

Class 5 Switch

• A carrier-grade switch used by telecom operators/ISPs to provide local phone service to subscribers.
• Software-based (softswitch) in modern setups runs on servers or in the cloud.
• Handles public voice services for thousands or millions of end-users.
• Connects directly to customer endpoints (home phones, IP phones, mobile networks, etc.).

A traditional PBX sits within a company’s premises, connecting internal phones and routing calls to the Public Switched Telephone Network (PSTN) for external communication. In contrast, a Class 5 Switch is operated by a telecom provider, delivering public telephony services to residential and business subscribers by connecting them directly to the PSTN and related services. Above that, a Class 4 Switch manages inter-city or long-distance call routing between exchanges, acting as the backbone for large-scale carrier-to-carrier communication.

Flow hierarchy
Internal Phones      →     Traditional PBX →   PSTN/SIP Trunk  →   Class 5 Switch →   Class 4 Switch → Long-Distance / Other Carriers