Conncet 3 asterisk server using Sip Trunk

I’m using Asterisk (Ver. on cent os.
I have 3 asterisk server A, B, C. Each work fine. Asterisk A is connected to both B and C using two Sip Trunks and it works fine too. The thing is How B can establish a call to C using sip trucks B->A than A->C.
I’ve looked everywhere but could not find answer. I’ll appreciate if anyone here tell me how?

Include the dialplan rules form making calls from A to C in the context for calls arriving from C.

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