I assume that you’ve setup the trunk and that is it registered according to this:
I have set up my freepbx server and have it registered with my vonage account.
Next step would be to setup an extension followed by setting up an Inbound and Outbound Route.
In the Inbound Route setup your Destination will be set as the extension you created. You will leave the DID and CID fields blank.
Make sure that you’ve opened ports on your router so that your external extensions can register on the server.
Only do the following if you DO NOT have a static ip address.
In Freepbx go to Admin, System Admin, DDNS
Check and see if you have an External DDNS Name. If NOT, you can go do NoIP and setup one that points to your IP address. I’m assuming that you do NOT have a static ip address otherwise you do not need to do this.
Now you can setup your softphone by using the DDNS name you found or setup as per above as the SIP Server address. Username will be your extension number and Password as set when you created the extension.