Hi Gene,
First of all, for API, SDK, or development related questions, those are probably better posted on the Genesys Cloud Developer Center Forum: https://developer.genesys.cloud.
That said, please visit the API Explorer on the Developer Center: https://developer.genesys.cloud/devapps/api-explorer and search for "trunkbasesettings". That should filter the list down the the GET, PUT, and POST commands available to manipulate a trunk.
It can also be useful to log into Genesys Cloud, bring up the browser's developer tools and watch the Network traffic when you make updates to a trunk. That will help you understand how Genesys calls those APIs.
Cheers,
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Jim Crespino
Senior Director, Developer Evangelism
Genesys
https://developer.genesys.com------------------------------