Response to OPTION and something abt subscribe

I setup a ondo sip server on a PC. When my softphone registered on it ,
constantly sending OPTIONs,the OPTION is like,for convenience,I have
ignored some content.
Request-Line:OPTION sip: 10.28.105.109:5060 SIP/2.0
Method:OPTIONS
Message Header
Via: SIP /2.0/UDP 10.28.102.76;rport;branch=xxxxxxxx
Content-Length:0
Call-ID:XXXXXXXXXX
CSeq:1 OPTIONS
Max-Forwards:70
SIP to address: sip:10.28.105.109:5060
from
to

And the ondo reply me a 404 not found .Truely the on do having the
address 10.28.105.109 as written in the Request_Line . and when I
register another server ,it reply a 480 temporily unavailable.I wonder
the exactly use of this OPTION and why server reply it as above

Another question,I would like to know something abt SUBSCRIBE .The
softphone send a SUBSCRIBE after a register request all the time ,but I
don't notice anything special in the content . I want to know when you
request some source where it will be annouced in the SUBSCRIBE.

The last ,I see when a phone registerring it will first send a
register request without AUTHORIZATION althrough username and password
are configed, if failed , send a request contain AUTHORIZATION. Someone
tell me where this behavior defined or say exactly.

Great Vincent