Trunk unregisters instantly after registration

I already configured four trunks (cisco spa8800) to asterisk and they works just fine, but when i tried to configure goip1 as trunk i see that goip unregisters after being registered for 1 second, what could it be?

What i see on my asterisk log:

-- Registered SIP '905' at [goip ip address]
-- Unregistered SIP '905'

My trunk configuration:

host=dynamic
username=905
password=password
type=friend
nat=no
dtmfmode=auto
qualify=yes
canreinvite=no
disallow=all
allow=alaw&ulaw&g729

My goip configuration:

Endpoint type:     SIP Phone
Config Mode:       Single Server Mode
Phone number:      905
Display name:      905
Authentication id: 905
Password:          password
SIP Proxy:         [asterisk ip]
SIP Registrar:     [asterisk ip]
Re-register period: 30

Why goip unregisters after registration?

Only goip can answer that, although you should note that you are using deprecated/obsolete options names, a value of type that is often found in cook book solutions, but is nearly always less secure and less reliable, and that you are expecting goip to authenticate themselves to you, which they are unlikely to do (see parenthetical remarks in the last topic I replied to).

Also note the remarks about the support status of chan_sip in my last reply on the forum.