1. Busy: If more than one person calls at the same time, the line may be busy. I suggest you try again later.
2. Signal problem: If the signal of your mobile phone is not good, it may affect the call quality. I suggest you go to a place with good signal and try again.
3. Operator problem: Sometimes network failure or operator maintenance may also lead to call problems. You can contact your operator for more information.
4. Incorrect phone number: Please make sure that the phone number you entered is correct, including the area code and every digit of the number.
If the above methods can't solve your problem, I suggest you contact Shanghai Public Transport Card Service Center for consultation and solution.