Invite error

I have freepbx server and application for calling over sip. The problem is, when i try to call someone(currently i call from one notebook to other), response for the first two or three invite requests will be 500 server error, and only then server will process the call request. I checked invite requests, they don’t change, and both notebooks are connected to the network and registered in asterisk. Is this behavior normal, if not, where i should look for errors?