bro iam too m31 user I have been using con call for past one month ther no problem in merg the calls may be ur service provider problem call the customer care of bsnl
Yes, your phone doesn't have any fault, it's the service provider. call customer care ask them, when I was using bsnl in 2012, then also bsnl wasnt providing conference.
I think there is something wrong in software of your device because i am also a M31 & BSNL user, it works fine on my device. Try to reach Samsung Customer Care
