So, first of all I suppose that you have a static ip address in your office.
You could try to proceed with a provisioning of the phone with this public ip address.
Currently, the phone is listed with a private ip address in the "Devices" and registered in "Users" with a public one!!!
If after this kind of provisioning you'll still have troubles, the only way to discover the problem is to take a pcap trace on the pbx and at least, a pcap capture on the remote side....