This is more of a workaround but try adding:
To sip.conf. It’s a new feature as of Asterisk v11
wiki.asterisk.org/wiki/display/ … +Keepalive
If it still does it try set [quote]qualify=no[/quote]
To the extension.
The keepalive=yes will take care of keeping the NAT port opened so technically qualify=yes is no longer needed if that is the case.
I personally think this qualify OPTIONS reply problem is a long standing Asterisk bug of some sort. If you do a search you see a lot of people having this problem randomly and not necessarily because they have a lot of extensions or certain brands/models of hardware. Messing around with configs doesn’t always solve it unless it involves workarounds