If you want to avoid getting a second call when you're busy in Teams, you can change your call settings. You can choose to send all incoming calls to voicemail, to another person, or to a call answer. You can also set a schedule for when you want silent mode on. To change your call settings, follow these steps:

1. Open the Teams app and click the 3 dots next to your profile picture in the top right corner
2. Click Settings, and then click Calls.
3. Under the Call answering rules section, you can choose what to do with incoming calls when you're busy on another call. Select Play a busy signal
   
4. Close the settings window.

When a person tries to call you, they'll see the following message: