1. The acoustic components of mobile phones are of poor quality, and the louder a poor Speaker is, the more obvious the noise will be.
2.. Aging telephone lines are a very common noise fault.
3. The metal of the telephone line or telephone interface is excessively oxidized or corroded.
4. The internal line of the telephone is aging or the chip is faulty.
5. Power interference.
6.. The radio frequency signal of the mobile phone itself will be strung into the audio signal to become noise, if the mobile phone design is not well filtered, there will be obvious noise when the mobile phone calls