How to connect SIP incomming call to FXO outbount


I’m recently deployed 3 FreePBX in different locations say A,B and C and all are interconnected with SIP.

All box are with FXO ports and lines are connected.

In the above scenario i need to call “A” PBX to call land-line phones through FXO in “B” PBX

Please help me to find the configuration

Thanks in advance.