I have just installed 3CX, licensed 12.5 system. I have Cisco SPA5xx IP phones. I've read and re-read the documentation on setting option 66. I found a DHCP testing utility and it shows what I have put in for option 66 is being returned, which is: http://10.10.1.7:5000/provisioning/y2j1i57kn0/$MA.xml I have checked out what is in Settings > Phone Provisioning and on the extension I am trying to put up. The phone had firmware 7.5.2 which I updated first to 7.5.2b and then to 7.5.5. I even tried the 7.5.7s version, but no joy. I've factory reset the phone. I have used this phone on earlier versions of 3CX, but don't remember having such trouble. It is as though 3CX is not returning anything when the phone tries to make connection to the URL. One twist is that the phone is on a different subnet connected via VPN, but the connectivity is very good and DNS and various protocols work fine site to site. Thank you for any assistance.