It looks like you are talking about registering an IP phone to the corporate Cisco CM cluster. This is not the channel for you to ask this. You should use internal channels in Cisco or open an IT case for these questions.
The short answer is you need to be VPN'd into the network (which would only be supported with an issued ECT router or via the VPN client running on a PC). From there, if the IP phone is behind the tunnel, you can get it added to the CM cluster as another phone with your DN (it will be a shared line). Or you can get them to add a CIPC instance, so that if your computer is behind the tunnel, you can use that.
No clue why you mentioned callcentric. That's a third party SIP service, and would work for inbound/outbound calls, but wouldn't give you access to corporate DNs, voicemail, etc.
Closing this thread, though, since this is not grounds for an external discussion.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.