How To Connect SIP Phone To FXO Line

I’ve got two HT-503s with their FXO ports configured and have the trunks, inbound and outbound routes setup. There are lots of pages out there talking about setting up trunks and I/O routes.

But nobody talks about how you connect your SIP phone to this setup.

At the moment I have my phones (Grandstream GXP2130s) setup with buttons to connect to the trunks. I get a dial tone and can dial out.

But shouldn’t these buttons connect to the outbound route somehow instead?