How can I call internationally from China?
Most bigger hotels have international long distance access from the phones in each room, but rates are generally quite high. Your other options are to bring an international phone card with you from home or try to negotiate buying one in China. If you are using a Chinese cell phone (or your own phone with a Chinese SIM card) you can add an international long distance feature for a few pennies and per minute rates are extraordinarily low. Both Chinese mobile phone service providers have an English customer service line you can call to request this service. No matter how you do it, make sure you know the country code for where you want to call. China's country code is 86.
Will my mobile phone work in China?
It depends on the type of phone. There are two main carriers in China: China Mobile uses GSM and China Unicom uses CDMA. If you have a dual-band or tri-band unlocked GSM phone, you can use it in China. You can also purchase a SIM card in China for use in compatible phones; this will give you a Chinese number and allow you to purchase minutes in the form of cards that may be purcashed for different values (RMB 50, RMB 100, RMB 500).
You can use your mobile phone with an IP card to make international calls. IP cards can be purchased for around RMB 30-35, though the printed value on the card is usually RMB 100 (be sure to push for a lower price if the vendor asks you for the full value on an IP card).