Changing sip port is not working


I have configured Asterisk@Now with default sip port 5060 @ sip.conf file. It works. But when i change it to 5988 in sip.conf also add line to user.conf to a specific user port = 5988. then its not responding.

Is it a bug??? or We can’t change sip default port 5060 to any???
If isp blocks 5060 then what should we do???

Please help.