from US t-mobile to use weChat etc in Mainland
If you occasionally visit China but your good old China phone is too old regarding battery, memory or other problems, you want migrating WeChat etc to a new phone. Say you happen to have a US spare phone to use in mainland China. 
Steps:
-Insert a China (mobile) sim (18$/month for the phone number lease; min activity required every quarter to keep the phone number active). The US phone prompts phone/network locked. Pick option unluck the phone temporarily to verify the phone does work with China (mobile) network. Note if you pick permanently unlock at this point, the screen will prompt “error 8: phone not qualified for permanent unlock”
-test by visiting eg baidu.com successfully 
-soon screen prompts temporary unlock is expired and permanent unlock is now available. Pick that option and unlock. 
-google playstore only works with US sim with data roaming in China. So download WeChat that way. No WeChat available for download from Samsung Galaxy app store; the WeChat duo 微信分身or 微信双开seems to need payable membership （so do this step first if your spare phone has a working US sim in China, or even better download WeChat when in US before visiting China)
-put China sim back in and login WeChat with wxid and pass. Follow the Prompt  to scan the barcode on the screen with the old phone which hosted the WeChat Acct. Login with China phone number with prefix +86 does not work. 
Good luck!