Asterisk is sending Option request like crazy

Hi, I installed asterisk 1.6 and registered my SIP client to the asterisk server. Then, the SIP client is receiving Option requests from the asterisk server every 2 to 3 second. Please help why. The below is my client setting in sip.conf,

[3001]
type=friend
context=default
host=dynamic
domain=192.168.1.35
disallow=all
qualify=yes
allow=ulaw
rfc2833compensate=yes
caninvite=no
Thx,

voip-info.org/wiki/view/Asterisk+sip+qualify

Cheers.

Marco Bruni
www.marcobruni.net