Yes, freepbx is hosted on the cloud and different location from clients, and yes the server has a public IP directly assigned to it.
Yes, just for testing I adjusted one phone’s registration time to every 10 sec.
I don’t see any REGISTER request from the second ISP, they’re all showing from the first ISP, still.
I can confirm that the secondary ISP is whitelisted. And yes, I can confirm that SIP requests are being send out by phones to the server, I did a packet capture with wireshark with a compuer on the same network as phones.