[Users] logon problem

I have set an openser in a cs department with almost 50 sip phones connected to it. It is a very heavy loading environment. Every sip phone may make/receive almost 20-40 calls per minute. In such situation, most of the sip phones will experience a logon problem. They will failed to logon (in the display of the sip phones show wait logon) and 1 minute after, they can logon to the system. This situation will repeat for every sip phone in the system. I wonder what is capacity of the openser. As I expect, it should handle over 1000 sip devices connected concurrently (correct me if I am wrong). I also wonder any misconfigure in the configuration file. I set children to 4 in the file. Do I need to increase the children value? Is it useful for solve this situation? If not, what is the possible action to solve the above problem?