Class 4 vs Class 5 Softswitch: What’s the Difference?

A Class 4 softswitch routes calls between carriers — it is a high-volume transit switch for wholesale VoIP. A Class 5 softswitch serves end users directly, providing extensions, voicemail, call forwarding and other subscriber features. Class 4 moves traffic; Class 5 delivers services to people.

The terms come from the old North American switching hierarchy, but they are still the clearest way to describe what a VoIP platform is built to do. This guide compares the two, explains where a session border controller fits, and shows where a mobile gateway platform sits in the picture.

Where the “class” names come from

The Bell System organised its long-distance network into five classes of switching office. Classes 1 to 4 were toll and tandem centres, from regional hubs down to local toll offices, that moved calls between areas; Class 5 was the end office, the local exchange connected directly to subscribers’ lines. When the public network moved to IP, vendors kept the vocabulary: a softswitch that does transit work is “Class 4”, and one that serves subscribers is “Class 5”.

Mobile networks describe the same split differently. In an IMS core defined by 3GPP, subscriber features live in application servers, while interconnect and breakout functions handle traffic to other networks. The underlying idea is the same: carrying calls between networks is a different job from serving the people who make them.

What is a Class 4 softswitch?

A Class 4 softswitch sits between carriers. It receives calls from customers who buy termination, chooses a supplier route for each destination and passes the call on. It does not know or care about individual users; it deals in trunks, prefixes and rates.

• High throughput — measured in concurrent calls and calls per second (CPS).
• Least-cost routing (LCR) — choosing the cheapest eligible supplier that meets quality targets for each prefix.
• Quality-based routing — failover and rerouting driven by ASR, ACD and PDD per route.
• Number translation — normalising dialled numbers and prefixes between partners.
• Interconnect billing — rating each call twice, once against the customer’s rate and once against the supplier’s cost, to calculate margin.
• Partner management — per-customer and per-supplier trunks, capacity limits and credit limits.

What is a Class 5 softswitch?

A Class 5 softswitch is what a business or residential user actually talks to. It registers phones, rings extensions and provides the features people expect from a phone service.

• Subscriber features — extensions, voicemail, call forwarding, transfer, conferencing, hunt groups.
• Hosted PBX and IVR — auto-attendants, queues and time-based routing through an IVR.
• Numbering — assigning DIDs to users and meeting emergency calling requirements.
• Retail billing — plans, bundles and per-user invoices.
• Endpoint management — provisioning desk phones, softphones and WebRTC clients.

An on-premises PBX such as one built on Asterisk or FreePBX is, functionally, a small private Class 5 switch. Hosted platforms offer the same features to many tenants from a shared core.

Class 4 vs Class 5 side by side

Many real platforms blur the line. A hosted PBX provider may run its own small Class 4 layer to route outbound traffic across several carriers, and some wholesale platforms add limited retail features. The labels describe primary purpose, not strict boundaries.

Routing, LCR and CDRs

Both classes produce CDRs, but they use them differently. A Class 4 platform uses CDRs to calculate margin per route and to measure supplier quality; a falling ASR on one route is a signal to move traffic. A Class 5 platform uses CDRs to bill subscribers and to show call history. Good telecom billing depends on accurate answer and end times either way.

A note on least-cost routing

Pure cheapest-first routing tends to push traffic onto the lowest-quality routes. Well-run Class 4 platforms combine cost with measured quality and apply compliance rules, so that only licensed, contractually authorised suppliers are eligible for a destination. Our explainer on wholesale VoIP call termination covers how that market is structured.

Scale and media handling

The two classes also scale differently. A Class 4 platform is often signalling-heavy: many designs keep the switch out of the media path entirely and let RTP flow directly between carriers, so a single routing cluster can handle very high call rates. A Class 5 platform usually has to touch the media — for voicemail, recording, conferencing, music on hold and IVR prompts — so capacity planning is driven by concurrent calls and transcoding load rather than calls per second. That difference is why the two roles are so often run on separate systems, even inside the same operator.

Where the SBC fits

A session border controller sits at the edge of either kind of switch. It is not a class of switch itself; it protects and normalises the traffic going in and out.

• Security — topology hiding, access control, rate limiting and protection against SIP scanning; see securing SIP.
• Interoperability — fixing header and signalling differences between vendors and carriers.
• NAT traversal — anchoring media for endpoints behind firewalls; see NAT traversal.
• Media — optional transcoding and encryption termination (TLS, SRTP).

Product categories

Without recommending specific vendors, the market broadly divides into:

• SIP proxies and routing engines — open-source projects such as Kamailio and OpenSIPS are often used as the core of Class 4 style routing.
• Media and feature servers — Asterisk and FreeSWITCH provide Class 5 features, IVRs and conferencing.
• Commercial wholesale switches — integrated routing, rating and partner management for carriers.
• Hosted PBX and UCaaS platforms — multi-tenant Class 5 services with web portals and apps.
• SBCs — dedicated appliances or software at network borders.

Where a GSM gateway platform sits

A mobile GSM gateway platform is neither a Class 4 nor a Class 5 switch. It is an access layer: it presents mobile lines as a SIP trunk to whatever switch you run. Most commonly that is a Class 5 system — a company PBX that needs local mobile lines at a branch, or a hosted PBX offering mobile presence to its tenants.

GSMCalls gives that switch a managed trunk (UDP, TCP or TLS, with SIP ACLs), routing to sites and device groups, CDRs, prepaid minute profiles and multi-site management. It is intended for lawful, authorised use of mobile lines under the operator’s terms, not for carrying wholesale transit traffic; international traffic belongs on licensed interconnects. Start with what a GSM gateway is, or see the features overview.