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= If you have dynamic IP, use current for testing
localnet= eg 192.168.1.0/255.255.255.0

If the problem is still there then:
Check qos of your router in order to tune settings for VOIP.