I just found this out myself, SO strange that they would not integrate such an important option that was given to developers as part of iOS 10 and above. It's part of the RC Phone app, so guess we gotta leave both apps on the phone for this to work right?
Even more nebulous is that Android devices don't even have such an option (within ANY RC app) even though it's OS also supports a "callkit-like" functionality.
I've opened tickets about this and the support team doesn't even seem to comprehend what I'm even asking for. But using any VOIP app is useless if a carrier-call can put an active VOIP call on hold without even asking the user. This used to be an OS limitation (both Apple & Android) but they fixed it YEARS ago.