This app uses UICC Carrier Privileges to override VoLTE/VoWiFi settings in phone by overriding appropriate Carrier Configuration. More information on the following page - https://source.android.com/devices/tech/config/uicc !!! MUST HAVE !!! 1. An USIM/ISIM with an ARA-M applet having application identifier (AID) on the card A00000015141434C00 and the following SHA-1 certificate of this app. You may update these rules/certificate through card over-the-air (OTA) updates or push them manually if you have programmable version of USIM/ISIM. SHA1: E4:68:72:F2:8B:35:0B:7E:1F:14:0D:E5:35:C2:A8:D5:80:4F:0B:E3 2. VoLTE/VoWiFi capable phone 3. SIM card placed in first SIM slot of the device (only for multi-sim capable devices) 4. Make sure the SIM PLMN is this list https://android.googlesource.com/platform/packages/providers/TelephonyProvider/+/master/assets/carrier_list.textpb Instruction to provide Carrier Privileges to CoIMS can be found in following link: https://github.com/herlesupreeth/CoIMS_Wiki