In addition to what my colleagues already mentioned, check whether your LAN has DHCP Option 66 enabled.
If this is true, the phone will try to follow option 66 before it follows the normal provisioning procedure 3CX uses (causing the failure as a result).
Also, make sure the MAC you entered in 3CX is the correct MAC the device has. If there was a typo, the phone will never try to provision.
In case the above was the cause of the issue, add everything again from scratch and then factory reset the phone after this is done. It should come up, take a minute to contact RPS, and show a login prompt on the LCD screen for you to enter your credentials.
Hope this helps!