asterisk users - Oct 2010 - Send INVITES and REFERs from OpenSIPS to Asterisk with multiple Contexts

I currently have OpenSIPS set up with users and most of my call handling.
OpenSIPS won't be able to handle things like Call Park, Hunt Groups, ACD,
etc. So I want to send these types of requests to Asterisk. I also want to
set Asterisk up as Multi Tenant. So my question is

How can I send requests to Asterisk and have them funnel into the specific
context for that specific Tenant? So if customer from blah.com wants to park
a call then the users phone sends a REFER message to OpenSIPS, OpenSIPS
forwards that REFER to Asterisk and Asterisk sees that its from blah.com so
it does the does the context in dialplan that is set up for blah.com

I see that you can set up SIP Trunks and tell that SIP trunk that whatever
comes from the IP of the OpenSIPS server will be in context X, but if all
messages will be coming from that Single OpenSIPS IP how can I differentiate
between Contexts?

Any help or docs are appreciated.
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