Since conference calling is a service provider feature, I can only suggest a few things you can try.
-Clear the phone app data
-Reboot the phone
-Reset Network Settings
-Reboot device into "Safe Mode"
I hope this bit of info helps in any way.
I have the same problem, I have bought Note 10 plus, I am unable to merge calls.
I have spoken with some friends on the same network as me who have different phones, and they are able to do it. I have also spoken with a few friends with Samsung on different networks to me, and they are not able to do it.
Therefore I find it very strange that you put this on the network? Perhaps you should look more into what is happening with the phones.
Please advise as I could really do with using this feature in my line of work, and since buying this phone it has not been possible.
I am having the same issues. Up until a month ago I was able to make conference calls . I called Samsung and they had me to reset my network and the merge button reappeared.