We recently experienced a problem trying to register a Snom 710 phone to a 3CX server.  Whatever we did the phone refused to register.  The phone appeared within the Phones console via PNP but couldn't be automatically or manually provisioned

We eventually determined that the problem was related to the client using a public ip addressing scheme for their internal network.  In this instance the LAN was 199.x.x.x

There were two tasks required to resolve this issue:

1. The IP address of the phone was listed in the IP Blacklist within Security section of the Settings of the 3CX console.  The IP address needed to be removed

2. Most notably we had to untick the attribute 'Disable use of extension outside the LAN' on the Other tab of the Extension Details

A bit of a 'gotcha' which won't be missed again!