Which SIM should I choose for my 4G GSM Intercom?
TLDR:
Videx, AES, Commtel (Priory Access before 2022)... anything but Vodafone (VOXI, Asda Mobile, Lebara, and Talkmobile are also Vodafone)
Priory Access (2022+) anything but O2 PAYG until the end of 2025. Then any network, any SIM!
In the 2G/3G days you would chose the SIM that offered the right amount of calls for the lowest price. So long as you have network coverage you'll be fine. That is still valid... but only if the service aligns.
2G and 3G use have a direct digital voice channel to the (Circuit Switched) telephone network; just like old analogue phones. 4G phones were designed for digital voice networks and have no native voice ability. Just like a VoIP phone 4G calls use an internet connection for voice calls.
However, mobile internet is just not good enough to make lag-free high quality voice calls so the networks have a special connection they provision for low latency, low data rate voice calls. You videos and emails will go through a regular internet channels but your calls will go through a separate connection provided by your network.
When 4G (also known as LTE) was first released this was not implemented as it continued to be develop into the system which would become known as VoLTE (Voice over LTE). In around 2015 the system standard was set and this aligned with LTE release 10. However, further changes were to be made to VoLTE up to release 13.
4G calling? You drop back to 3G or 2G for calls!
4G Calling, Networks and Phones
It's 2025 and all UK networks now support VoLTE calling. However, not all devices and SIM cards can make 4G calls. Firstly your device has to be new enough. On Vodafone you need an LTE.Rel12 or later device. On 3, EE & O2 a LTE.Rel10 is sufficient.
Any phone after 2020 unless it's the cheapest pile of junk imaginable will work on Vodafone but older phones may not. They may work on the other 3 networks that support the older standard but they won't be as reliable as a fully rel12+ device.
When you make a call on one of these old phones you will see the network change from 4G to H (3G), 2G, or Edge (also 2G). This is because they lack the ability to make a 4G call with your network.
It's not just the phone, O2 still do not allow 4G VoLTE calling on any of their PAYG SIMs including Tesco PAYG and any GiffGaff SIM. Remember this, this is important!
4G Intercoms
Priory Access LTE Release 13 Intercom
Most 4G intercoms will use a 4G modem, which is like an entire phone but in a solderable block the manufacturer can solder to their main PCB. As there was little choice to start with almost all 4G intercoms older than 2022 will use a modem based on the Qualcomm MDM9207 chips. These are LTE release 11 chips and support VoLTE... but no on Vodafone.
There is also another consideration. These chips do not enable VoLTE as standard. It has to be explicitly turned on which some manufacturers do not do (we're talking about cheap Chinese gear here really).
Videx 4G Intercom used for VoLTE testing
Modems which feature the MDM9207 chipset include the SIMCOM SIM7600 and the Quectel EC21/EG95. These modems are found in the follows intercoms amongst others:
- Videx 4G
- AES 4G
- Commtel 4G
- Priory Access (priory to 2022)
These chips offer 2G, 3G and 4G. Whilst Vodafone still offer 2G, these intercoms will work fine with any SIM from any network. However, it's likely they will be unreliable on Vodafone as they are dependent on 2G which is not well maintained anymore. Furthermore, the early VoLTE is not quite as reliable as the rel.12 onwards so can cause intermittent reliability on all 4G networks.
EG915U by Quectel
In 2022 Priory Access switched to Unisoc chips which offer VoLTE support on all networks including Vodafone worldwide. However, these chips do not support 3G as the technology is being phased out globally and will be gone entirely in the UK by 2026!
Whilst these chips allow Priory Access to future proof their intercoms, they do have issues with O2 PAYG SIM cards. The above GIF shows my phone, with GiffGaff PAYG SIM dropping from 4G+ down to 3G to make a call. When a call comes in, O2 will attempt to connect via 3G to make the call. That's fine with my phone as it supports 3G. However, our up-to-date rel.13 chipset in the Priory intercoms doesn't. So incoming call can fail in areas where there are still 3G masts.
With these chips we would recommend getting an O2 contract SIM such as Tesco Contract. Towards the end of the year this problem will fade away and this chipset will eventually be without vices. But, for now you should choose any network apart from O2 PAYG in modern rel.13 equipped intercoms.
