Did you check the permissions for the KV6113 when you shared it with your other user?
When you share a device from one account to another the sharer can set the features that the secondary user has access to, in our experience access to all permissions is set as default but we have never shared an intercom with another user so it may be different and the call function may be disabled by default in the permissions list.
If you go to More > Manage Sharing Settings on your app and check the permissions you will see what the KV6113 is currently set at.