Asterisk acl.conf

/etc/asterisk/sip.conf
[general]
contactacl=example_named_acl1

or on  specifiy sip peer

[102]

contactacl=example_named_acl1

/etc/asterisk/acl.conf

[example_named_acl1]
deny=0.0.0.0/0.0.0.0
permit=209.16.236.0
permit=209.16.236.1