3CX Phone Server, Secondary Addresses and PortForwarding

Hi, all- a small quandry. I am attempting to set up a 3CX phone server for a customer. The WAN's primary IP is already established at xx0.xx0.xx0.241 255.255.255.248. The client has a few more IPs at their disposal, so I thought I could add a secondary address (xx0.xx0.xx0.242) to the same WAN interface. The phone service and server(s) would then be set up to exclusively utilize this secondary address. Phone traffic would be aimed at the .242. Rules would then be set up to forward traffic from the .242 WAN to a 192.168.4.13/24 server. Piece of cake, right?

I added the .242 secondary address to the customer's WAN interface. Created Services and FW rules to forward the traffic. (I have done this a few times before. Pretty sure I have this part down pat.) Also, and importantly, opened the server firewalls to this traffic, too! :)

But, here's the thing- I start up the 3CX phone server and run it's builtin FW checker. The checker logs details that the traffic is coming from the .241 IP, not the .242 IP that is currently configured in just abouyt every place I can find. Internal and external DNS, the phone server itself- all there to receive VoIP goodness from the .242 address. But, that is not happening- 3cx logs say traffic is trying to be be forwarded from .241.

So, am I missing anything? Many thanks...