Adding phone to user's account using Duo Admin API

Hello,

We are building an in-house application, using Duo Admin API calls, for the users to validate themselves using some secondary parameters and then add their phone number.

What is best way to validate user’s phone with Duo Admin APIs? For example user enters phone number on the form, we want to sms them a verification code and have them enter it back on the form, then associate the phone to Duo user. Can similar functionality can be done with the Admin API? I have tried calling
/admin/v1/phones/[phone_id]/send_sms_activation but this doesn’t return the same code that user sees on the phone app, so I can validate it.

Any help appreciated,
Thank you!