The setting I mentioned should exist if you have a "Multicast Paging Group" created under "Ring Groups". Based on the subject of your post, you are using Multicast Paging. Is this not the case?
The Yealink setting you mention does not sound like a setting related to this issue plus I would not recommend making manual configuration to the IP Phone as this would not only render the configuration unsupported but would also probably not be a permanent fix since 3CX provisioning may very well overwrite the changes.
In case I misunderstood, could you please clarify on what the issue is exactly and what relevant configuration currently exists on your 3CX Instance(Mutlicast Paging Group, Paging Group, etc)?