When user complain that her external incoming calls goes directly to voicemail but received calls from people using Teams..

Cause: User is using Microsoft Teams for calls, user profile is set for Skype for Business.

Solution: User profile need to upgrade to Microsoft Teams mode only.

How do I find out if the user which mode Skype for Business or Teams.

The best way to find out is to open Skype for Business and make test inbound call, if call receive user is in Skype mode.