Here's my config:

Settings on the N900:

User name: 5005@192.168.0.82
Password: [secret]

Enabled is checked

Advanced:
Use for telephone numbers is checked
User Name: Not set
Transport: Auto
Outbound Prody: Not set
Port 5060
Discover public address disabled
Autodected STUN disabled


Asterisk configuration:
SIP.CONF
[5005]
type=friend ; This device takes and makes calls
host=dynamic ; This host is not on the same IP addr every time
username=Nokia ; Not really used
secret=*****
nat=no ; nat=yes if this phone is behind a NAT box or firewall
context=home ; My internal context for phones
insecure=very ;bypass authentication
qualify=30000

EXTENSIONS.CONF:

exten => 7005,1,Dial(SIP/7005,25) ;
exten => 7005,2,Hangup ; and then hangup.