Let’s Talk About Voice in Private Mobile Networks
The private mobile networks (PMN) market continues to grow steadily into 2025. As 5G Standalone (SA) products mature and integration processes become more efficient, adoption is picking up speed. Although there’s still a learning curve, the market is clearly moving forward — and with it, we are observing consistent patterns across projects. One trend in particular stands out: the growing importance of voice services.
Voice may not be the first requirement brought up in a PMN project, but almost inevitably, the question comes: “How about voice?”
Voice is a fascinating topic because mobile networks were originally designed with voice at their core — enabling people to stay connected while on the move. Voice may seem simple, but it is technically demanding. Even though voice requires a relatively low bitrate compared to video, it is extremely sensitive to bit errors and jitter. Changing codecs mid-call is also highly disruptive. In fact, in professional media productions, voice is often considered more complex than video because the human ear is more sensitive to variations than the eye.
When it comes to delivering voice in private networks, there are two main approaches:
- Voice over LTE (VoLTE) and Voice over New Radio (VoNR)
This is the traditional method, using the phone’s native calling application. VoLTE and VoNR rely on the IP Multimedia Subsystem (IMS), which separates voice traffic from data by creating a dedicated, high-quality data flow for each call. In public networks, this separation is critical for handling congestion. In PMNs, where network dimensioning can be tightly controlled, it’s less about congestion and more about ensuring predictable performance. - Over-the-Top (OTT)
Voice OTT voice services — like Messenger or WhatsApp calls — treat voice as regular data traffic, using protocols like SIP (Session Initiation Protocol) to connect devices. This approach requires a dedicated application for making calls but offers flexibility and often quicker deployment.
Voice Services Beyond Basic Calls
Private networks often need additional voice services such as group calls, push-to-talk, and voice-command features. These can be implemented independently of whether VoLTE/VoNR or OTT voice is used. However, choosing the right architecture can simplify (or complicate) future expansions.
Connecting to the Outside World (RoW)
Another question that often comes up: “How can I connect my private network to the outside world?”
- IMS-Based Approach:
Integrating with an existing PBX (Private Branch Exchange) requires syncing user data between the PBX and the network’s HSS/UDM and establishing a SIP trunk. This setup enables the addition of advanced services like group calls and supports mission-critical applications. We’ve seen this approach widely adopted in public safety deployments. - SIP-Based Approach:
If using OTT voice, integration happens between the PBX and the SIP client. Assuming the PBX vendor offers a compatible client, this can be a straightforward solution. However, adding complex services later might require custom development, as there is less standardization behind these setups. - No PBX Scenario:
Without a PBX, using IMS would mean the private network would act like a public network, with voice relying on roaming — practically impossible in most private deployments.
Multi-Site Considerations and Roaming
In LTE, roaming between sites can be tricky. The practical workaround is to provision users at each site. However, 5G brings significant improvements: using SEPP (Security Edge Protection Proxy), roaming becomes seamless, even when using SIP-based voice.
Additionally, in 5G networks, APIs allow dynamic changes to QoS settings, making it possible for SIP-based voice services to achieve a level of quality similar to IMS-based voice — something not widely exploited yet, but opening new possibilities.
This is where 5G private networks come into play. When optimized for OT use cases, 5G can provide:
Alternatives: VoWiFi and DECT
- VoWiFi (Voice over Wi-Fi):
SIP-based voice services can also run over Wi-Fi networks. While Wi-Fi is excellent in offices and homes, it struggles with mobility and interference in more challenging environments. Still, it’s a strong complement to mobile networks, especially since Wi-Fi operates in unlicensed bands. Authentication via SIMs and IMS integration can help bridge Wi-Fi devices into the voice system securely. - DECT (Digital Enhanced Cordless Telecommunications):
DECT is a proven technology, widely used in industrial and office settings. It integrates easily with PBX systems but is limited to voice (with very little data capacity). DECT operates in unlicensed spectrum but offers a simple, stable solution where high mobility isn’t critical. - Other Specialized Solutions (like P25):
Technologies such as P25 are popular in specific sectors (e.g., public safety) and share similar trade-offs: established ecosystems but limited bandwidth for additional services.
Conclusion
Voice is a technically challenging but critical service for many private mobile network deployments. There is no universal “right” way to deliver voice — the best approach depends heavily on the specific environment and operational needs.
In many cases, it might even make sense to stick with existing voice solutions and focus private network resources on data applications first. Regardless of the chosen path, understanding the options — and their trade-offs — is key to building successful voice capabilities in private mobile networks.
