[Users] logon problem

[unplug]

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?