[Asterisk-Users] Confusion with IAX PBX-PBX

[Chris Lee]

I have been trying to set up three * servers to use IAX between them and 
am  a bit lost as to the finer detail of the config files. I have read 
the wiki and it has not made things better.
Here is my problem;

I create a section like this on each machines:
[othermachine-1]
type=friend
host=dynamic
secret=password
trunk=yes
qualify=yes
context=incoming-1

[othermachine-2]
type=friend
host=dynamic
secret=password
trunk=yes
qualify=yes
context=incoming-2

Now in my extensions.conf I use the link like this:
IAX2/othermachine-1

But my problem comes in with the receiving machine, how does it know
which machine the link came from without a username of some kind.

Or have I completely missed the point of IAX?

Please help I am completely lost.

Thanks

Chris.