is it possible to have FS forcilby unregiter a registered UA, for example, user/1001?
Yes, try the following: sofia profile default flush_inbound_reg user@domain Found this using: 'sofia help' in the freeswitch console. So you can find all specific commands using this help (it's not documented in http://wiki.freeswitch.org/wiki/Sofia-SIP).
to go one step further, how can i do the unregister upon receiving another register request?
I'm not sure what you're trying to achieve, but the wiki on sofia-sip gives some documentation.
i checked event list of the event socket but cannot find any register event, can anyone shed any light on this?
In my console (logging into it with fs_cli -n) it's shows register events and I believe this is the standard configuration: [WARNING] sofia_reg.c:1533 SIP auth challenge (REGISTER) on sofia profile 'internal' for [xxxx@xxxxxx] from ip XXXXXXXX. -- EL