Outbound calls issue

I have the following setup
[PBX1 with phones] - trunk - [FreePBX 11, no phones here] - trunk - [SIP provider]

Just installed a fresh version of Freepbx, configured the trunks between systems. FreePBX says trunks are online.

Now what I’d like to achieve is that from PBX1 location, where I do have a desk telephone to call through SIP trunks a local number through the SIP provider.

What should I do ?

Connects the two FreePBX with IAX and then the outputs of routes to local network!

how to assign a DDI to an extension afterwards ?