Mobile Data in China, Hong Kong, Macau
I was just on a study trip in December 2015 that visited China, Hong Kong and Macau. Staying connected to the Internet via 3G mobile data is a little tricky, especially in China. Short answer: get something from China Unicom HK. If you are stopping in HK, get it there, otherwise get it online directly from www.cugstore.com or from an online reseller.
September 2017 update: if you are based in Singapore, you should get a $15 StarHub Happy Prepaid card and activate the Happy Roam service. Currently, you get 2.4 GB / 30 days for S$15 (of the S$18 credit on the card), and it covers many other countries too, apart from China, Hong Kong and Macau.
I was just on a study trip in December 2015 that visited China, Hong Kong and Macau. Staying connected to the Internet via 3G mobile data is a little tricky, especially in China. Here's how!
What's the short answer?
Short answer: get something from China Unicom HK. If you are stopping in HK, get it there, otherwise get it online directly from www.cugstore.com or from an online reseller.
For voice and data, get the Cross Border King Dual Number Prepaid SIM, and for data only, get one of the Data SIM options that match your travel plans.
I got a Greater China 1GB 30-day Data SIM (which could receive SMSes on a HK number) because I underestimated the utility of a local mobile phone number in China; I highly recommend that you get a voice and data SIM.
Here's the long answer.
Why China Unicom?
China has three big telcos: China Mobile, China Unicom and China Telecom. Due to national regulations, all SIM cards require ID to be presented for registration, so you'll need your passport and perhaps an address.
For mobile data, you really want some form of 3G at least. Currently, according to the prepaid-data-sim-card wikia site, (WebCite archive), only China Unicom's 3G network is compatible with most phones from outside of China, as it runs HSPA+ on 2100 MHz. Check GSMArena.com to make sure that your phone supports this!
For 4G/LTE, the situation is a little different, I didn't investigate it so much. I think that China Mobile's TDD-LTE network works at least with some iPhone 6 models for the Chinese/Asian market. Both China Unicom and China Telecom are also said to support FDD-LTE on 1800 MHz, but a few minutes on Google didn't yield much about the actual deployed coverage. YMMV. This guide covers things from a 3G perspective. (Maybe I should have Bing'ed or Baidu'ed.)
Why China Unicom HK?
Let's say you settle on China Unicom (www.10010.com). However, there are some quirks to deal with Chinese telcos. Apart from registration, you must note that the telcos are organised provincially. Therefore, plans are often split into "local" (within province) and "roaming" (outside province) voice and data usage, with different quotas and rates. Also, there are said to be issues topping up cards across provinces!
It's easier to sidestep these issues entirely and just deal with China Unicom HK instead. No registration, no provincial quota problems, no topup issues since they take VISA/Mastercard internationally via an online portal.
Where do I get a China Unicom HK card then?
Some sources online say that you can buy these at HKIA (the airport), but believe me, we tried everywhere in the secure area and couldn't find it. Not the Travelex'es, not the PageOnes, not the electronics stores.. some of them will have the China Mobile local or cross-border cards in stock, but that's about it. You might have better luck at the public area if you are actually entering into HK.
It's best to pick these up from some street retailer in HK if you are stopping over there before heading into China, as they're said to be available for cheaper than SRP on the street. Otherwise, you can try to look for it online on eBay etc., but availability is a bit sparse.
Instead, you can buy direct from China Unicom HK at www.cugstore.com, and pay their extortionate HKD 50 flat rate international shipping that is actually untracked international airmail letter. I got a HKD 50 coupon for signing up during some promo which offset it, but I suggest you buy early since they're untracked and there doesn't seem to be any option for tracked/express shipping – they didn't respond to my emails asking about that. My envelope arrived with a handwritten address and HKD 5.50 in stamps. (Alternatively, find a friend or a parcel forwarder in HK! Local courier shipping is free or HKD 20.)
Anything special about these SIMs?
There's also a bonus effect: the internet access is uncensored, since everything is being tunneled back through to the "home" network, which is China Unicom HK.
Interestingly, China Unicom HK is actually a MVNO in HK, running on some combination of 3HK and csl, so the SIM card may be permanently "roaming". But hey, it's still considered a home network, and Facebook and Instagram work perfectly fine, so no complaints.
One thing to note: I seemed to be getting 2G speeds in Macau (on 3 Macau) but I didn't have time to check that out beyond rebooting my phone a few times. It could simply have been due to severe network congestion in the town centre, seeing as it was a Sunday afternoon. YMMV.
Also: the data SIMs really don't have voice at all. You can't even call customer service, so better have some Skype credit handy if you think you might need it.
Why get a voice and data SIM?
If you don't want or need voice, you can get a Data SIM. These may be better value per gigabyte, and come in a bewildering array of options: Hong Kong & Macau, Guangdong & HK, China & HK, or Greater China (China, Macau, Hong Kong and Taiwan), and all for different data quotas and durations. For my Greater China Data SIM, it operated on 3HK, roamed onto China Unicom in the mainland, as well as 3 in Macau. It's also listed as connecting to Chunghwa in Taiwan.
However, after a mere 5 days in Hangzhou and Shanghai, I strongly recommend that you (or at least someone in your group) get a card with a voice number on it. A local number means you can use local services like Didi Kuaidi (to get cabs) as well as get queue numbers at restaurants. Given that the queues at popular restaurants can be easily 1-2 hours or more, it can be quite useful. Of course, you'll still be fine without a voice number, just a little constrained.