Architecture

Class 4 vs Class 5 Switches (Trunking vs PBX)

Class 4 and Class 5 are inherited from the original AT&T 1950s switching hierarchy, but the distinction still defines how modern softswitches, SIP trunking carriers, and PBXs are built. Class 4 = wholesale transit (trunk-to-trunk routing, billing, no end-user features). Class 5 = end-office (extensions, IVR, voicemail, transfer, hold). Most products today are explicitly one or the other, and trying to use a Class 5 system as a Class 4 carrier (or vice versa) is one of the most common architectural mistakes in VoIP.

The original AT&T hierarchy

AT&T's 1950s long-distance network was a 5-class pyramid:

• Class 1: regional center (national long-distance backbone, ~10 in the US)
• Class 2: sectional center
• Class 3: primary center
• Class 4: toll center / tandem (long-distance switching)
• Class 5: end office (CO — central office serving subscribers)

By the 1990s the hierarchy collapsed into mostly two practical roles: Class 4 (transit / wholesale) and Class 5 (subscriber-facing). Modern VoIP inherits this vocabulary, even though softswitches don't physically resemble the old TDM equipment.

Class 4 (transit / softswitch / wholesale)

A Class 4 system routes calls between other carriers. It does not serve end users.

• Inputs: SIP trunks from upstream carriers, VoIP peerings, TDM trunks.
• Outputs: SIP trunks to downstream carriers or Class 5 systems.
• Features: least-cost routing, rate decks, CDR billing, traffic shaping, fraud detection, STIR/SHAKEN signing/verification, codec transcoding, NAT traversal at scale.
• Does NOT have: extensions, voicemail, IVR, hunt groups, ring tones, hold music, BLF/presence.
• Capacity: tens of thousands to millions of concurrent calls per node.

Examples: Kamailio (route logic), OpenSIPS, Cataleya, Sonus PSX, Oracle ECB, Sansay VSXi, Ribbon NSX. Almost every wholesale SIP trunking carrier (DIDHub included) is a Class 4 stack at its core.

Class 5 (end-office / PBX / hosted PBX)

A Class 5 system terminates calls at end users and provides telephony features.

• Inputs: trunks from a Class 4 / carrier; user registrations from phones, softphones, Teams.
• Outputs: calls to extensions; calls back out to the trunk.
• Features: extensions, IVR, ring groups, hunt groups, queues, voicemail, voicemail-to-email, transfer, hold, hold music, conference, BLF/presence, CDR per user.
• Capacity: hundreds to a few thousand registered users per node.

Examples: Asterisk + FreePBX, 3CX, FreeSWITCH (configured as PBX), Cisco CUCM, Microsoft Teams Phone (with Direct Routing), RingCentral, Zoom Phone, Avaya Aura, Mitel.

Side-by-side

	Class 4 (trunking)	Class 5 (PBX)
Primary purpose	Route calls between carriers	Serve end-users with features
Who registers to it	Other carriers / softswitches	End-users (phones, softphones)
Has extensions?	No	Yes
Has voicemail / IVR?	No	Yes
Per-call billing?	Yes (rate decks, CDRs)	Often per-user, not per-call
STIR/SHAKEN signing?	Yes (originating attestation)	Rarely — relies on upstream
Codec transcoding?	Yes, at scale	Limited, per-call
Capacity	10K-1M+ CPS	100-5K registered users
Typical software	Kamailio, OpenSIPS, Sansay, Sonus	Asterisk/FreePBX, 3CX, Teams, RingCentral

Where DIDHub fits (Class 4)

DIDHub is a Class 4 carrier. We do not sell hosted-PBX features — no IVRs, no extensions, no voicemail. We provide DIDs, SIP trunks, STIR/SHAKEN, OBR-aware least-cost routing, and managed SBCs (for Teams Direct Routing).

You bring your own Class 5: an on-prem PBX (Asterisk, 3CX, FreePBX), a hosted PBX (Teams, Zoom, RingCentral via BYOC), or an AI voice agent platform (Vapi, Retell, ElevenLabs). DIDHub is the wholesale telecom layer underneath.

Common architectural mistakes

• Trying to use Asterisk as a wholesale carrier: Asterisk is a Class 5 PBX. Above ~500-1000 concurrent calls it falls over — per-channel state, voicemail processes, etc., all inflate memory. For trunking at scale, switch to Kamailio or OpenSIPS in front (Class 4) and keep Asterisk only for media-anchored features.
• Trying to use Kamailio as a PBX: Kamailio has no native voicemail, IVR, or queues. People who try to bolt these on inevitably end up running an Asterisk behind it — correctly recreating the Class 4 + Class 5 split they were trying to avoid.
• Buying 'a SIP trunk' when you needed a hosted PBX: a SIP trunk gives you a phone number and minutes. It does not give you 'press 1 for sales'. You need a PBX (or AI voice agent) on top.
• Buying a hosted PBX when you needed wholesale termination: if you're a SaaS reselling voice, RingCentral / 3CX cannot give you the per-minute economics or DID inventory of a Class 4 carrier. Go direct to a wholesale provider like DIDHub.

Hybrid: when one node does both

FreeSWITCH is the most common Class 4/5 hybrid. It can run as either, and large deployments use it as both: a Class 4 dispatcher in front, Class 5 PBX cluster behind, all the same software.

This works for mid-sized telcos but not at carrier-grade scale — once you exceed ~10K concurrent calls per node, splitting into purpose-built Class 4 + Class 5 nodes becomes mandatory for performance and operational sanity.

References

• ITU-T Q.700 series — Signalling System No. 7 (SS7) network architecture
• RFC 3261 — SIP (Class 4 and 5 both implement this)
• PBX (Class 5)
• SIP Trunk (Class 4 product)

Related terms