Can I buy/rent a cheap phone in China to use just while visiting for 2 weeks?

My cell phone is an old prepaid one that works only in USA. Along with many other international attendees (from Europe, Asia, USA), I will attend a conference in Macau for two weeks, with some of our time spent in Hong Kong and mainland China. We must be able to call each other without roaming, and also to stay in touch with the local Chinese organizers. Is it true it could be cheapest for all to buy phones in HK or Macau - for use only during our convention - and just donate the phone to a local charity when we are leaving?