[Users] system busy - scenario

I have experienced an error with using openser 1.0.1 + mediaproxy 1.4.2. The system works fine in the following loading. User login to the system: 20 hardware phones concurrent RTP session: 5 However, when I increase number of user to 50 and concurrent RTP session to 30. After a few minutes, the system is very unstable. SIP phones display "wait logon" message as they can't logon to the system in some reasons and no one can make call even some users can logon. This situation will remain few minutes and it will resume normal. After a while, system will become unstable again. Anyone has such experience before? I have grepped the log using ngrep but it is very difficult to troubleshoot as it is a log of whole system with 50 users. In my simple analyst, I found that the most error replies happened in the log are listed below. 407 - proxy authentication required 408 - request timeout 481 - call leg transaction does not exit Do above errors will make the system unstable? Which one is the most critical? When and how the above errors will occur? To repeat the situation above, it involves a lot of resources. Is there any sip phone simulator which can be used to simulate as a large number of user to create calls simultaneously? Thanks!!