Yeah it only works with phones within the same subnet of a self-hosted 3CX server. The TTL is also set so it won't propagate through routers either unless you do some packet mangling (eg. to use PIM-SM or similar to broadcast across subnets).I've read that multicast paging doesn't work with a 3CX hosted server.
Founded in 2005, when VoIP was an emerging technology, 3CX has gone on to establish itself as a global leader in business communications.