I used to think it was easy to change a phone card, and it would be better to throw it away if you didn't need it. However, with the word "reputation" becoming more and more important, it seems that unused phone cards need to be written off, even though there is a rule that the balance in the phone number will be automatically recovered after 90 days. Since I have a mobile phone, there should be more than ten (maybe fewer) phone cards used by China Mobile, China Unicom and China Telecom