Viva-MTS, Armenia's largest mobile operator, offers its subscribers the ability to make VoLTE calls now. VoLTE enables calls over their existing 4G/LTE network. It is touted to provide higher quality voice connections, and connection setup times of just about 1.5 to 2 seconds. It also promises stable connection with minimal latencies, and does not require users to stop using the Internet during a call.
In order to make VoLTE calls, users will need a VoLTE compatible smartphone with the latest software updates, a Viva-MTS 4G/LTE USIM or eSIM card and network compatibility, and VoLTE connection setting.
Users can check their VoLTE status by sending the command *412#.
Source: https://www.thefastmode.com/technology-solutions/33738-viva-mts-offers-subscribers-volte-calling