Mobile networks are more modern than they have ever been. In Sweden, we have said goodbye to 3G for good, and 2G remains only in a limited form – and only for a short while longer. Globally, the situation looks largely the same. But with modern networks come new questions, and in connection with these shutdowns two issues stand out in particular.
The first is that SMS can no longer be sent from IoT hardware once GSM has been shut down. The second is that voice calls via VoLTE do not work in applications such as gate controllers or access systems, even though the device itself claims to support VoLTE.
Why your IoT device stopped calling
For a long time, voice calls in technical installations have been something few people needed to think about. SMS and voice services simply worked, relying on 2G or 3G, often without anyone being aware of which underlying technology was actually used.
When operators shut down the legacy networks, both SMS and voice services change fundamentally. There is no longer any possibility to continue using circuit-switched telephony, neither for voice calls nor for SMS. Instead, all voice and messaging services must run over 4G or 5G – with VoLTE acting as the common enabler.
The result is that many legacy IoT systems, based on older devices —devices that have “always worked” —suddenly can no longer make calls or send SMS, even though the hardware on paper supports VoLTE.
How voice and SMS worked in 2G and 3G
In older mobile networks, both SMS and voice calls were circuit-switched services, completely separate from data communication. When a modem or device registered to the network, it automatically gained access to SMS and voice services via a dedicated channel, independent of any data session. These networks carried a legacy from analogue telephony, meaning that no IP connection was required for voice calls, no special modem configuration was needed, and the SIM card only had to have basic telephony enabled.
What changes in 4G and 5G
In LTE and 5G networks, there is no circuit-switched telephony provided. All SMS and voice calls are delivered as IP traffic via VoLTE. VoLTE, just like modern SMS delivery, is based on IMS – the operator’s IP-based service platform. It is easy to assume that VoLTE is purely a modem capability. In reality, VoLTE is an operator service where several components must work together. The hardware must support VoLTE technically, the firmware must correctly handle IMS registration, and the subscription must be provisioned to allow VoLTE for the specific type of device.
A common misconception is that VoLTE itself is a requirement for sending SMS in modern mobile networks. What is actually required is that the device is allowed to register with IMS, the IP-based service platform of the mobile network. VoLTE is one of the services using IMS for voice calls. SMS in LTE and 5G networks is delivered via the same IMS platform, but as a separate service.
For IoT installations, this means that the SIM card and the subscription profile must allow IMS registration, even if the device is never intended to make voice calls. If IMS registration is blocked, data communication will work as expected, but SMS delivery may fail completely.
The subscription trap
When it comes to subscriptions, this is where things often break down. In the network sunsets we have seen in Sweden, the root cause is very often that the SIM card simply does not have these services enabled. The modem registers only for LTE data and never performs IMS registration. This becomes particularly visible in IoT deployments. Many subscriptions are designed purely for data communication and therefore allow LTE data traffic, while blocking VoLTE and other IMS-based services. The result is a device that appears to function normally in the network, while SMS and voice services never connect.
Hardware can still be part of the problem
Many established LTE modems used in industrial routers were developed during a time when 2G and 3G were still available. These modems may handle SMS through fallback to legacy networks, but in some cases lack a complete IMS profile for SMS over LTE in the standard firmware. Based on our experience today, however, the vast majority of routers and modems delivered to the Swedish market in recent years do support SMS over LTE. That said, individual installations and markets may of course differ.
A necessary reset for connected systems
The shutdown of 2G and 3G is not a temporary disruption. It is a structural change in how voice and messaging work in mobile networks.
For installers and system owners, the takeaway is clear: future-proofing is no longer just about radio technology or coverage. It requires a deliberate check of the entire chain – modem capabilities, firmware maturity, IMS support and, just as importantly, how the subscription is provisioned.