iphone caller to Ring Deskphone cannot merge call

When person calls my user from an iphone and then wants to Merge (conference) another call. As soon as they hit Merge, the call to the Ring Deskphone drops immediately.

This seems like it could be related to :
https://community.ringcentral.com/questions/93312/cannot-merge-calls-iphone-error.html

This is super frustrating. I tested this on multiple ios devices with different versions and it happens on all of them. How could this be ? I have never heard of a system in which conferencing calling is effected in this way.