With the growth of private 5G Standalone (SA) deployments, enterprise communication architects face the challenge of merging new mobile infrastructure with existing VoIP networks. According to the Ericsson Mobility Report 2025, over 90 operators have launched or are in the soft-launch phase of 5G Standalone. The goal of enterprise-level integration is not just to provide physical connectivity between domains, but to create a unified Least Cost Routing (LCR) contour and unified billing.
Without architectural convergence, an enterprise ends up with two isolated systems. This leads to fragmented billing data, traffic management complexities, and critical vulnerabilities in subscriber identification at the technology junction. The main problem lies in the incompatibility of signaling and billing protocols: traditional SIP systems use static trunks and generate offline CDR files, while 5G Core requires real-time charging and operates on HTTP/2 REST APIs.
Anatomy of the gap: why legacy SIP cores do not understand 5G Standalone
The integration problem stems from a fundamental difference in architectural approaches. Traditional corporate VoIP networks are built on the SIP protocol (RFC 3261), which operates over UDP or TCP. Session routing in such networks relies on static IP addresses or centralized SIP proxies.
In contrast, the 5G Standalone (5G Core) is designed using a Service-based Architecture (SBA), standardized by 3GPP. All network functions (NF) in 5G Core interact as microservices via HTTP/2 REST APIs. Instead of standard SIP trunks, dynamic service registration (NRF) is used. When a legacy system expects a standard INVITE/OK/ACK message exchange, the 5G core requires interaction with Session Management Functions (SMF) and the User Plane Function (UPF).
Without an intermediate convergent gateway, direct signaling interaction is impossible, which blocks end-to-end LCR and reduces overall Quality of Service (QoS).
Convergent LCR architecture: combining 5G and fixed VoIP
Least Cost Routing in a hybrid environment must account for not only external PSTN provider tariffs but also the availability of local 5G resources. To implement such an LCR, it is necessary to configure trunking between the 5G core and the corporate VoIP switch (softswitch). In 3GPP standards, this interaction is typically provided via the IMS subsystem or simplified MGCF/IP-SM-GW gateways.
In practice, this works as follows: when a subscriber with a 5G smartphone dials an external city number, the LCR module analyzes the routes. Instead of exiting through a commercial mobile operator's network, the call is routed through the corporate PBX's local SIP trunk into the city network at a significantly lower rate. For effective operation, the module must support dynamic MNP (Mobile Number Portability) analysis and determine the optimal exit point in real time.
Unifying billing: moving from CDR to real-time charging
Traditional corporate VoIP billing is offline: after a session ends, a CDR (Call Detail Record) file is generated, which is later processed by the billing system. However, 5G Core standards require the use of a real-time Charging Function (CHF) and a Convergent Charging System (CCS). This is because transaction volumes are increasing sharply: scaling of 5G subscriptions is expected to reach 6.4 billion by the end of 2031 (according to Ericsson forecasts), which also includes a massive number of IoT devices.
The solution is to implement the principles of the TM Forum Open Digital Architecture (ODA). Moving to a componentized, API-first architecture allows for unified billing. According to observations from integration projects, about 53.7% of organizations face difficulties synchronizing billing cycles between domains, and up to 27.7% of the implementation budget can go toward customizing legacy interfaces without using Open APIs. Real-time billing allows for processing both 5G traffic and SIP calls in a single cycle, instantly reacting to the exhaustion of corporate user limits.
Security at the environment junction: call authentication (RFC 8224)
A private 5G core provides a high degree of protection (3GPP AKA, encrypted SUCI). However, when a call exits into the SIP domain, there is a risk of Caller ID spoofing. The problem of source authentication is critical: global losses from telecom fraud are expected to reach $41.82 billion in 2025.
5G does not automatically solve the problem of VoIP trunk security. A reliable architectural approach is to apply the RFC 8224 standard (Authenticated Identity Management in SIP). When routing a call from 5G to a corporate contact center, the Session Border Controller (SBC) generates a cryptographic signature and places it in the Identity header of the SIP request. The receiving platform verifies this signature. If validation is successful, the system guarantees that the initiator's number is genuine, which minimizes social engineering risks.
Designing the target architecture: transitioning to a convergent core
To bridge the gap between SIP and SBA, enterprises need intermediate infrastructure with protocol translation support. One such solution is the carrier-grade VoIP platform DooxSwitch — a development from the portfolio of the Intecracy Group technology alliance. The solution works as a high-performance softswitch with an API-driven LCR module and built-in real-time billing, allowing session charging based on CHF/CCS principles.
To handle large datasets, complex integrations, and ensure strict access control, the platform can rely on UnityBase mechanisms (the low-code platform is a joint development of Intecracy Group companies; InBase acts as a key, but not the only, developer). UnityBase provides RBAC/RLS tools, deep audit trails, and the ability to automatically generate REST APIs for transparent integration between the telecom core and external corporate BSS systems.
| Comparison criteria | Legacy VoIP (SIP) | 5G Core (SBA) | Convergent solution (DooxSwitch) |
|---|---|---|---|
| Signaling protocol | SIP (RFC 3261) via UDP/TCP | HTTP/2 REST APIs (JSON) | Convergent gateway with protocol translation |
| Charging method | Offline CDR file processing | Real-time Charging (CHF/CCS) | Unified BSS contour via TM Forum Open APIs |
| Identification and security | Simple Caller ID (P-Asserted) | 3GPP AKA and encrypted SUCI | Cryptographic SIP Identity signature (RFC 8224) |
| LCR management | Static tables in Softswitch | Dynamic service selection (NRF) | API-driven LCR with MNP support |
A successful transition to convergent telephony requires an audit of existing SIP infrastructures, the deployment of API gateways for interaction with the 5G Core, and mandatory configuration of cryptographic call verification to eliminate vulnerabilities at the junction of the two domains.
FAQ
How do I configure LCR between a corporate PBX and a private 5G Standalone network?
Configuration is performed by creating a trunk between the corporate convergent softswitch and a gateway (e.g., MGCF/IP-SM-GW) in the 5G Core. The LCR module analyzes routes in real time, directing calls along the most efficient path to avoid unnecessary interconnect costs.
What requirements does 3GPP impose on integrating external SIP trunks into 5G Core?
3GPP requires adherence to Service-based Architecture (SBA) principles. External SIP signaling must be translated into HTTP/2 REST APIs using media and signaling gateways to ensure interaction with Session Management Functions (SMF).
How can real-time billing be implemented for 5G subscribers using a corporate VoIP gateway?
This is achieved by integrating the corporate BSS system via TM Forum Open APIs. As a result, VoIP calls and 5G data sessions are charged synchronously through a unified Charging Function (CHF/CCS), allowing for real-time limit management.