Once you have your FreeSWITCH setup then adding new trunks should work fine but you can get some first-day-of-use problems that slow you down.As I find them I will update this post.

Calling PSTN number of DDI trunk, rings then silence.
This is probably due to the trunk on FreeSWITCH not having the  “Register” checkbox set (true). If using bluebox then in the Connectivity->Trunk Manager for the trunk check the Register checkbox. In XML this is in the conf/sip_profiles/ in your .XML file for the profile of the interface you need to verify that register is true,

       <gateway name="trunk_x">
        <param name="realm" value="sip.example.com"/>
        <param name="username" value="xxxxxxxx"/>
        <param name="password" value="xxxxxxx"/>
        <param name="register" value="true"/>
        <param name="extension" value="auto_to_user"/>
        <param name="auto_to_user" value="true"/>
      </gateway>

Calling PSTN number of DDI trunk, rings then very quickly goes to disconnect.
This is due to the inbound ACL not being set to “Trunks (Auto)” on the Sip interface (NAT).