About the amazing RedPocket plan (great for use in China, and also for calling China from the US):
[Updated 2022.10] I received this SIM card in September (purchased in late August, so it took less than a month). It did not require customs clearance; I just picked up the package at the post office. It can be activated while in China. To avoid spam calls, I transferred in a nice Google Voice number. After activation, enabling Wi-Fi Calling requires contacting online Chat support on the website.
eBay purchase link:
Activation notes:
After purchase, you will receive an activation code by email. It is very important, so make sure to save it, because you will need it later for activation.
During activation, you need to enter the code you received by email (mentioned above). If you cannot find it, search your email for Red Pocket Activation Code.
To enable Wi-Fi Calling, contact the chat support in the lower-right corner of the website. Once they finish the setup, just restart your phone.