In smartphones like Nokia 7 plus and Huawei P20 Pro you have dual volte standby which means currently only single sim can use VoLTE that is your primary sim, but in these phone you can use VoLte in both of the sim that is Primary as well as Secondary
Thank you for reaching out to the Samsung community! We regret to inform you that Note 8 does not have dual Volte support. Our development team will review it and if feasible we'll try to implement. Keep Posting!
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