So you’re using https://marketplace.zoom.us/docs/api-reference/phone/methods#operation/getPhoneRecordings to get the download url and then when you use it, the format is HTML? Do you see any message on the page?
The following apply to your situation: Downloading a video from the url returns soft 401 html page - #5 by zoom-test
This thread also has some helpful troubleshooting steps based on your configuration: Zoom Cloud recording “download_URL” working only in browser - #11 by tommy
Best,
Gianni