I considered this example. 408 is a code of California so probably this example is for “usual phone number”.
I would like to double check: does “usual phone number” value really contains no leading ‘+’.?
If yes: how to distinguish it from “extension”(second case from my question)?
No, the phone number value can contain a leading ‘+’. For instance, if the caller is a non-zoom user, the leading “+” plus will be included in the API response. I can see how this can be a bit confusing, so I’ve submitted an internal ticket to our documents team to update the description to mention the difference between the extension number and e164 numbers [DEV-2408]. If you would like to directly provide your user story to be taken into account, I recommend submitting it to: Feedback - Zoom
Also, I should note that the caller_number_type property is another indicator that can distinguish if the phone number contains an “extension” as well. If the value is 1, then it is most likely an extension. While if the value returned is 2, it is an external number, so the phone number is likely a “usual phone number.”