The raison d'etre of any voice intercom is, unsurprisingly, to make voice calls! It can have any number of fancy features and apps but without the ability to make calls it's useless. Unfortunately, the majority of intercoms on the market today have a real problem with this.
2G and 3G have a dedicated voice and SMS channels as part of the connection to the network which connect calls directly to the circuit switched analogue phone network. 4G has no native voice channel and does its calling through the internet in the same way as a VoIP call does and this is called Voice over LTE (VoLTE). However, the usual 4G data connection would not be good enough for delay free calls that can be passed from tower to tower so it requires a special connection through a dedicated low-latency data channel. The whole calling and SMS system on 4G is handled by a software system called the IP Multimedia System (IMS).
To understand the issue with 4G intercoms, we must define what 4G is. The specification for mobile standards is handled by the 3GPP organisation and they have released 22 separate standards for mobile as of 2021. Clearly, as you can't buy a 22G mobile phone, some of the commonly recognised evolutionary steps (i.e. 2G, 3G 4G, 5G) must have multiple releases.
The first 4G release was the 3GPP release 8 specification in 2008. The first VoLTE specification was included in release 10 in 2011 which is the first time voice calling was able to be made on 4G using a phone. Very few networks around the world implemented this early version of 4G calling and most networks continued to use the 2G and 3G connections using something called Circuit Switched Fall Back (CSFB). The phone may operate on 4G for the internet but calls were made using 2G and 3G. This continues to this day (August 2023) on many networks in Europe that haven't yet implemented VoLTE/IMS.
Vodafone UK did not offer 4G calling until after 2018 and only for iPhones. Mobile phone networks do not retrospectively invest in old technology so the minimum specification for 4G calling on these networks is 3GPP release 12 which was an earlier release at the time. This can be found in the Vodafone Device Requirements document (https://www.vodafone.com/content/dam/vodcom/files/suppliers/device-requirements-compliance-framework.pdf). They state their IMS system is compatible with 3GPP release 12 and later (https://www.gsma.com/newsroom/wp-content/uploads/IR.92-v12.0.pdf).
So for 4G calling on all UK networks we're going to need a phone or intercom with a chipset that meets at least 3GPP release 12 spec. This is where the problem lies.
Cellular intercoms use a self-contained circuit called a modem which is effectively a mobile phone within a little metal can that can be soldered to circuit boards. The first 4G modem chipset that offered VoLTE calls was the MDM9207 released in 2016 and this conforms to 3GPP release 10/11 (depending on version). This chipset is used in the SIMCOM SIM7600 modem which is the basis of almost all 4G intercoms including those made by Videx, AES and Commtel/Comelit (the main UK intercom brands). This modem is used because of its in-built audio codec making it easy to design into an intercom.
Our testing on Videx intercoms reveals that the modem falls back to 3G with Vodafone UK SIMs as they don't conform to the rel.12 standard. When 2G and 3G networks are shutdown these intercoms will no longer work on these networks and the SIM card should be swapped to a 3UK SIM. EE and O2 do support the primitive IMS found on rel.11 chipsets, however, 4G calling has been unrelibale on these networks with that modem which is why we recommend 3UK and not EE. This is the case for every intercom featuring the SIMCOM modems.
Both the budget version and the IQ versions of Priory Access intercoms are based on a chipset released in 2019 which conforms fully to the 3GPP release 13 standard and offers 4G calling on all networks throughout Europe. Priory Access Intercoms do not have this problem. Similarly, the 8915 chipset we use in our intercoms is an LTE Cat1.bis chipset which means it has been designed for just 1 aerial. The earlier MDM9207 is designed for 2 aerials which it never gets which degrades performance further.
Priory Access Intercoms are available here.