[Asterisk-Users] outbound proxy field in sip.conf

[David John Walsh]

I have been given the following settings for connecting to a voip
provider.  The names of the fields match my snom phone, and when
configured, the phone both makes and recives phonecalls without issue.

I am trying to put the same values in asterisk, but there seems to be
one field that doesn't seem to exist in asterisk - that of outbound
proxy

all suggestions welcome
SIP headings
account    = user
password  = secret
registrar   = host  = registrar.provider.com
outbound proxy = ?? = nat.provider.com:5065

If I put in an extra field of port=5065 it doesn't register (in sip
show registry) with either of the above addresses in the host box. 
Putting registrar.provider.com in the register => string and
nat.provider.com in the host makes outbound calls fail, putting
registrar.provider.com in the host and nat.provider.com (with or
without the :5065) in the registrar doesn't allow incoming or outgoing
calls
putting reistrar.provider.com in both host and the register => string
allows outgoing but not incomming

firstly, what does the outbound proxy do?  secondly, can anyone advise
on settings for this senario? does asterisk have this concept in its
SIP client?

Thanks

David