Dial extension on system B from IVR on system A

Good day, I have multiple Asterisk system connected together VIA IAX trunks.

I have a primary Gateway that connects to our SIP PSTN this also has our main IVR, when someone calls in to the Gateway (SystemA) they get our AA, and would like to dial an extension on SystemB, how do I make the IVR aware of these extensions or allow this DN range to be dialed from the IVR?

(Currently an SystemA is allowed to dial extensions on SystemB in both directions)