I have found error in sipsettings module in file page.sipsettings.php. There is superfluous comma in line #860 after curly bracket. That’s why module sipsettings doesn’t work fully in the Internet Explorer. For example, you have no chance to set additional local networks.
Could you submit a ticket for it? You can do it here: http://www.freepbx.org/trac/newticket