There are two types of LTE handsets:
Type 1: Handsets which are capable of sending both voice and data over the LTE network, and -
Type 2: Handsets which use LTE to only consume data, but the voice communication remains on 2G/3G.
In technical jargon, Type 1 handsets are known to be Voice over LTE (VoLTE) capable. These phones implement the GSMA IR.92 and IR.94 standards which define the characteristics a VoLTE handset must support.
Type 2 handsets are also referred to as “Data-only” LTE phones. Such phones, even though being LTE capable can use LTE 4G only for data. For some handset manufacturers, it is possible to “upgrade” such phones through a firmware update to VoLTE phones. However, this is not true for all handset manufacturers.
Backend Network -
Just because a phone supports VoLTE, it does not mean that voice will always go over the LTE packet network.
The 4G packet core network has a network element known as the MME (Mobility Management Entity). When the phone tries to “attach” to the network, the MME sends back an indication to the phone that alters its behavior.
There is a VoPS bit (Voice over Packet Switched), which when set to “1”, tells the phone that the network is also VoLTE capable, and voice can be sent over LTE. However, if the phone does not support sending voice “as packets” over LTE (Type 2 decribed above), then voice services will not work over such a network. Type 1 VoLTE phones would work well when the VoPS bit is set to 1.
Some networks also set the “CSFB Indicator”. CSFB stands for Circuit Switched Fall Back. This informs the handset that the network has a 2G/3G circuit switched network as a “fallback” for voice services, and hence voice calls can use the legacy 2G/3G network, while data may continue over LTE. Hence, Type 2 data only phones are best suited for networks which have CSFB.
I hope this clarifies sufficiently.
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