[SR-Users] Session authorization just before initiate it

Hi All in the list, I'm new to Kamailio but I'm actually able to quickly setup a server, including also the SIP presence, looking at the example I found at http://kb.asipto.com/kamailio:presence:k31-made-simple which explain very well how to do it. I also made some changes in the final setup and everything seems working well, I can have audio/video/chat session including the presence status of any user. Now I would like to improve my testing setup so that for each call request Kamailio receive, I would like to perform some SQL lookup for deciding if the conversation might be established or not. About the SQL lookup I read about the SQLOPS module, it seems really easy to use, but I don't know where to put such check before the INVITE. Could someone give me some hints? P.S: Indeed, I need to read some good documentation about how to code a Kamailio config file. Any URL to suggest? Thanks in advance, Roberto Fichera.