Kamailio is what is called a Session Border Controller. Its primary purpose is to provide protection and some lightweight filtering for the media servers/PBXs behind it. Once you want "advanced" features like voicemail, parking, hunt groups, three-way calling, etc, you will need to use a PBX like FreeSWITCH (recommended) or Asterisk (not) behind it anyway. If you're only running a single machine, an SBC isn't really worth the trouble.

> Once you want "advanced" features like voicemail, parking, hunt groups, three-way calling.

Right, but what I am saying is that I don't want any of those features. At least not yet.

I just want to give my customers a sip address (username@communick.com) where they can call other SIP addresses. In that case, is that a PBX still needed?

You certainly can use Kamailio but it will be much easier/faster to get something going in Asterisk.

You don't need to run any of the advanced features in Asterisk (and can completely unload the modules if you want).