OnePlus India — call conference is not working

Address:

560056

I write with great disappoint about the call conferencing issue which is being faced by me since more than a month time. I have visited the service center almost 6 to 8 times and was given various solutions, like as to change sim, upgrade software and others. Despite doing all these and even a complete reset done with new software updation this problem persists.
My last visit to service center was on 18th of may, where the technicians at center did a volte enablement and removed true caller app, post which it worked fine and now seems to continue with the same issue.
Since, call conferencing is a must need for my business calls, this issue has made be handicapped and impacting my work. Hence, i need this to be sorted out immediately or will need a replacement od handset or refund of the cost, whichever is viable.

Hoping for a complete resolution on this.