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.
I have come across this case a couple of times where for some reason after operating successfully for some days or weeks all port 80 traffic starts getting blocked by a 2wire router. This occurs when the router is connected to the computers through a switch, as opposed to all computers connecting directly to the.