To prevent incoming calls to a personal line the user will need to change their status to Do Not Disturb.
https://support.microsoft.com/en-us/office/change-your-status-in-teams-ce36ed14-6bc9-4775-a33e-6629ba4ff78e#:~:text=To%20explicitly%20set%20it%20yourself,Away%2C%20Offline%2C%20or%20Available