Thank you for reaching out to the Samsung community! We would like to inform you that call conference feature depends upon the network service provider. We suggest contacting the same network service provider to fix the call conference issue.
Be sure to hit ♡ when you find an answer that works for you. For further assistance, you can get in touch with us via Live Chat option by following- http://livechat.support.samsung.com/Customer_new/IN. You can also connect us via Samsung Members web community by following mentioned url-https://community.samsungmembers.com/en_IN#/
