FreePBX sip trunk with SBC/CUCM

Hello, as the title says, my configuration is as follows.

I have one CUCM server that is routed/ trunked through an SBC (cisco 4331) within a LAN.
I have a FreePBX server with FreePBX that is outside the lan, in an extension network which is permitted inside the initial LAN.
I have tried to make the trunk in the FreePBX with the external ip of the sbc. In the sbc as well, i have made the dial-peer and all required ip configs with the ip of the FreePBX server.

Still, i have no success in making calls either way. The 8841 ip phone does not return Call cannot be completed or dialed as it does in other invalid calls, instead it returns nothing, no tone, blank.
I know the topology might be a little complicated, but any help is appreciated, thanks.