How to use <user>@<domain> instead of <user>@<ip> ?

[jolouis]

I think you're mis-understanding the purpose of domain names. From a general perspective their purpose is simply to give "human readable/understanable names" to IP addresses on a network/the internet.

When SIP devices communicate with each other, they always use IP addresses for the actual packets sent back and forth. If you configure a domain name for the server address, all that means is that the phone will resolve the domain to it's corresponding IP address before sending packets out.