Thank you so much for your reply. Below I pasted the configuration for one endpoint. Can you check if the configuration is ok? I have to use only one video codec?
[transport-udp]
type=transport
protocol=udp
bind=0.0.0.0
[1000]
type=endpoint
context=internal
disallow=all
allow=ulaw
videosupport=yes
allow=h263p
allow=h264
allow=vp8
transport=transport-udp
auth=1000
aors=1000
[1000]
type=auth
auth_type=userpass
password=1000
username=1000
[1000]
type=aor
max_contacts=1