Hello Han,
Here's an update. Can you try setting the conversionTypeId to -1? That should uncheck the box in the AdWords user interface. We're currently working on updating the documentation.
Cheers,
Nadine, AdWords API Team
if conversionTypeId is set to null, the system-wide default conversion tracker for calls from ads (conversionTypeId 179) is used. If it is set to -1, phone calls to that number will not count as conversions (corresponding to "Count calls as phone call conversions" being unchecked in the UI).