No IP address has not changed, the IP address showing in nslookup is our main internet connection and different ISP to the one we use which is dedicated to the 3CX PBX.
If I lookup the FQDN in MXtoolbox the correct address is resolved, also we use Monitis to monitor our servers including 3CX systems and that is resolving by FQDN and can see the phone system.
I didnt reinstall, just restored from backup, yes probably did contain license key information.
We now think we have identified the problem, we setup a backup PC for 3CX on Friday which was just a Debian install of 3CX designed to be just to be backup hardware to restore to if needed.
A backup had been restored to this PC as a test however the backup device was sitting on the wrong subnet and therefore wrong internet connection and I suspect that restore on Friday has changed the public IP although this PC has been switched off since Friday.
How do I now get the correct public IP as it shows correct in customer portal and some DNS servers are resolving since the restore earlier but others are not.
I will maybe leave this until the morning to see if this propagates?