[Serusers] Limiting number of calls

[Waldo Rubinstein]

Is there a way to restrict the number of simultaneous calls for a  
particular sip client? For example, if I sell a service where each  
line can have at most 4 simultaneous call voice paths and there are  
two incoming calls and two separate outgoing calls, how can I make it  
so that whenever a new calls comes in or a user tries to make another  
call, they get fast busy or just busy?

Thanks,
Waldo