The Trunk Providers page can be used for commercial SIP trunk providers, for which E-MetroTel validated the configuration by interoperability testing. With the exception of Allstream, all the trunks in the provider list are registration based SIP trunks.
FXO gateways are neither registration based SIP trunks nor commercial SIP trunks. Due to that, you cannot really use the Trunk Providers page for FXO trunks provided via a SIP gateway.
To interface SIP gateways, it is necessary to configure a SIP trunk using the "PBX - PBX Configuration - Trunks" page.