If you are trying to get Acrobits SIP phone working on your iPhone with the 3CX VoIP telephone system use the settings indicated below. For the domain entered the local network DNS name of the server i.e. what is used to connect to 3CX on the local network. For Register via leave the default setting.
The instructions “How can I use the 3CX Tunnel with a SIP hardphone?” on the 3cx site at http://www.3cx.com/support/3cx-tunnel.html-