Hello, Our currently configuration of 3CX phone system with our ISP is working fine. Our manager has decided to change our ISP. So with our current ISP provide us with two public IPs , one for the Internet and the other for the SIP trunk, everything is working fine. However, our new ISP will provide us with one public IP for the internet and a private IP for the SIP trunk. My main concern is with the the remote phones (on laptops and mobiles). I don’t understand how it will be able to connect with our SIP server since we will be use Private IP instead of Public IP. I tried to make a test with our current infrastructure by creating a port forwarding of MIP the Internet Public IP point to private IP of our 3CX server so the remote phones will be using the Internet Public IP to setup a connection with our SIP trunk server. So, my 3CX phone for Windows has been connected successfully (ON HOOK) but The issue I came across is that from my 3CX windows phone I can receive calls but I cannot make calls with error message FORBIDDEN. Please i will really appreciate your help. I don't know if the above scenario is the correct way of configure your infrastructure when using a Private IP on SIP trunk. Thank you very much for your time.