Some SIP providers just work, and others require a little bit extra. If your trying to get ORBTALK working over SIP with your Asterisk 14 server then your going to need something like this:

host=sip.orbtalk.co.uk
username=1234567
secret=PASSWORD
type=peer
nat=yes
insecure=very
dtmfmode=RFC2833
allow=alaw&ulaw
port=5060
fromuser=1234567
fromdomain=sip.orbtalk.co.uk
context=from-trunk

Obviously change PASSWORD for your secret that you setup on their portal and 1234567 should be changed for the username from their portal.