From TS 24.301 :
EPS network feature support:
The network may include this IE in attach accept or TAU accept to inform the UE of the support of certain features. If this IE is not included then the UE shall interpret this as a receipt of an information element with all bits of the value part coded as zero. Note that this is an optional IE.
The EPS network feature support is a type 4 information element with a length of 3 octets and bits of third octet contains VoLTE feature support indicators as follows:
Bit 1 – IMS VoPS (IMS voice over PS session indicator)
0 – IMS voice over PS session in S1 mode not supported (VoLTE not supported)
1 – IMS voice over PS session in S1 mode supported (VoLTE supported)
Bit 2 – EMC BS (Emergency bearer services indicator)
0 – emergency bearer services in S1 mode not supported
1 – emergency bearer services in S1 mode supported
If IMS voice over PS session in S1 mode is not supported, MS may elect to utilize CS voice.
UE handling of this IE From TS 23.221:
If the UE is configured for Voice over IMS, the service domain selection functionality should take the “IMS voice over PS session supported indication” into account and should only initiate IMS voice calls (with the voice bearer in the PS domain) using the RAT where the “IMS voice over PS session supported indication” applies and indicates support.