Why Twilio Number Flagged or show Unknown?

  php, twilio

I Already Purchase some twilio number for calling purpose to my US client. after some days of calling my client told me that My Number shows as "UNKNOWN". That means my number is flagged now.

My question is

  1. Is there any way to know that my number is flagged/spam to my client?
  2. why this number is flagged/spam after certain time?

I can’t find answer or any link in twilio.

Source: Ask PHP