And any difference between restarting asterisk and reloading chan_sip? Thanks.
Restarting Asterisk will either break calls, or be delayed until there are no active calls.
sip reload command , will just reload your sip.conf configuration. Restart Asterisk will reload most of your configuration files not only sip.conf and also will break active calls.
There are three related commands for restarting Asterisk as well
core restart now - This command restarts the Asterisk service immediately, ending any calls in progress.
core restart gracefully - This command prevents new calls from starting up in Asterisk, but allows calls in progress to continue. When all the calls have finished, Asterisk restarts.
core restart when convenient - This command waits until Asterisk has no calls in progress, and then it restarts the service. It does not prevent new calls from entering the system.