Why do callers hear a busy tone when calling my phone from landlines?

[AC Question]

why do callers hear a busy tone when calling my phone from landlines?

I have a samsung galaxy s3. When callers call my mobile, they hear a busy tone all the time, but when called from any other mobile they can easily get through with the call. This has been a problem since I unlocked the phone and changed from t mobile to a sim from my country.