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 of domain
Here is the tricky bit. For the proxy enter the external address of your 3CX server on the Internet.
Change the RTP port start and Port End to the range you have opened in your firewall. Below is an example of the open ports in windows below