[Users] only one call from duplicate register

[Ibrahim TUNALI]

i know i can multiple register on a sip proxy with same account at same 
time from different locations. And both of them can call different 
numbers. But i want to only one client can call, other one will block 
during first one's conversation and second one will recive "not 
acceptable" response.
is it possible and can you give a configuration example.

regards,
itunali