Looks like a bug since only when using set paging and not multicast paging does it actually use the intercom feature. Just put in a intercom dial code and don't give the code out to people.
If you are using Yealink phones and Multicasting with 3CX give me a PM because you will find it works fine when you deploy it and then stops within 30 days. I can give you more of the back story and my work around.