In China there is GSM standard of cellular communication. Two companies: China Mobile and China Unicom offer cellular communication services in Shanghai. Of course you can use roaming but it is not well to pay so big many per one minute of telephone conversation.
If you plan to be in Shanghai at least one week I advise you to buy the Chinese SIM-card. Price of the card is 80RMB (10$) (about 40 minutes for local calls are included). At once together with the SIM buy a card of a IP-telephony (IP-telephony cards are produced by China Mobile and China Unicom too). You will speak with your country using these two cards. Talking with help of Chinese SIM-card and IP-telephony card is foure times cheaper than by roaming. You can buy the cards in any branches of China Mobile or China Unicom.
How to use IP-telephony cards
How to find out balance
During conversation you pay money from IP-card for the trunk call and from SIM-card for local call on access number (17950). Balance of the IP-card you can hear at each call. How to find out balance of the SIM-card? If you are owner of China Mobile SIM-card, send SMS on 10086 number. Write in SMS "11" (eleven). In the answer you receive a message. You will see blocks instead of the Chinese symbols, but figures are written normally. The second numbers (looking like cost) are the balance of your SIM-card. Of course if only your cellular does not support Chinese.
Now about cost of the IP-telephony cards. The card, that here is represented has face value of 100 RMB. But it does not mean, that you should buy it for 100 RMB. For an example, in Shanghai I bought them for 50 RMB, and in Longshan it was possible to bring down the price up to 40 RMB. Specify the price for the cards in your area at your translators or guides. It is possible to speak by IP-cards from call box. It's even cheaper, but with cellular is more convenient.