At trunk settings use the following:
context=from-trunk
fromuser=30211xxxxxx
username=30211xxxxxx
secret=xxxxxx
host=viva.gr
nat=yes
qualify=yes
type=peer
srvlookup=no
insecure=port,invite
canreinvite=no
dtmfmode=rfc2833
t38pt_udptl=yes
disallow=all
allow=alaw
User details
Leave it empty
Register string
30211xxxxxxx:secret@viva.gr/30211xxxxxxx
If the problem continues:
At /etc/asterisk/sip_nat.conf use the following settings
externip=
localnet=
If the problem is still there then:
Check qos of your router in order to tune settings for VOIP.