In my experience, Snom phones are very easy to set up and just work with their default settings. All you really need is the username, password and registration server and you're on.
If you can ring out, but calls are not coming in, check your firewall / port forwarding settings. Snom phones have pretty good logging, so switch it on from the web interface and make some test calls in and out. You can usually determine the cause of the issue by looking at the SIP traffic and searching google for the error message if it isn't obvious.