I can suggest some general methods to find out if a person is busy on another call:
Call Waiting Feature: Many phone services have a call waiting feature. If the person is on another call and has call waiting activated, you will hear a beep or a notification when you call them.
Voicemail: If the person's phone is not answered after several rings, it might indicate that they are on another call or otherwise occupied. You may be directed to their voicemail, where you can leave a message.
Text or Message: If the person uses messaging services like WhatsApp, iMessage, or other text-based platforms, you can send them a message asking if they are available to talk or if it's a convenient time to call.
Ask Them Directly: If you have an existing line of communication with the person, you can ask them directly if they are available to talk at the moment.
Check Online Status: If you communicate through messaging apps or social media, some platforms indicate the person's online status or if they are currently active.
Shared Calendar: If you have access to the person's calendar, you can see if they have scheduled meetings or appointments, which may indicate their availability.
Personal Assistant or Receptionist: If the person is a professional and has a personal assistant or receptionist, you can contact them to inquire about the person's availability.
Remember to always respect the person's privacy and not to be too intrusive when trying to find out if they are busy. If they are unavailable, you can leave a message or try contacting them at a more suitable time.