asterisk users - Jun 2011 - call transfer back to a sourcing switch

If call comes into PBX-A and based on the DNIS it comes into my box PBX-B
my box then says ring phone C. Person answers. They want to transfer the 
call
to a phone going back out PBX-A. All this is fine of course.

my question is when phone C transfers the call is there a way PBX-B can 
drop out
of the mix.

Jerry