Add a phone number as direct line in Zendesk API

Hello, 

I am using Zendesk Users API to create and update users. When I create a new user and add a phone number through the API, the phone is added as a direct line. If I update the same user and add another phone number, that is not enabled as a direct line. How can I enable the second phone number as a direct line through the API?

Thanks,
Asma