I have used Asterisk quite a bit.
Have you added your SIP phone to sip.conf? You're entry should look something like this:
[201]
type=friend
host=dynamic
username=201
secret=password
canreinvite=no
nat=yes
context=myphone
dtmfmode=rfc2833
allow=all
Then you just need a "myphone" context in extensions.conf, like:
[myphone]
exten => 102,1,Answer()
exten => 102,2,Playback(demo-echotest)
exten => 102,3,Echo
exten => 102,4,Playback(demo-echodone)
exten => 102,5,Hangup
Or you can just change the context= in sip.conf to a context that already exists in extensions.conf