unable to make and receive phone calls in Mainland China

I am going to assume you are using a physical SIM because eSIM's are illegal in China.  Roaming is very hit and miss in a communist country with very heavy censorship laws.  Do you have Roaming enabled in the settings on your device?

Thank you for your help. Yes, physical SIM, Roaming enabled from very beginning. Texted 611 but they didn't figure out after 3 hours text messages exchange. 
 

You really don't give us much to go on. What does work? What happens when someone tried to call you? What format are you using to dial numbers? Make sure WiFi Calling is turned off when testing your calls. Otherwise you'd be testing WiFi Calling and not international roaming..

Thanks- simply couldn’t make and receive calls

I had the same issue while traveling in China: not able to make or receive calls. Data and text still work. Many hours with T-Mobile support did not help. Tried all kinds of things such as reboot the phone, reset network and all settings, call #766# after manually selecting network between China Mobile and China Unicom. Finally figured it out after checking web forums and got some hint from my old working company phone on Verizon without 5G function.
 

The trick is LTE. For the calls to go through, the phone needs to enable VoLTE, and there should be strong LTE signal, and of course roaming needs to be on. If any one of the above conditions is not met, then the call not not be completed. 
 

The reason for this is that calls may default to 3G but 3G service is being phased out. And if you set to 5G, if will not work b/c T-Mobile's roaming agreement with Chinese carriers do not cover 5G. So if one does not force VoLTE on the phone or LTE signal is absent or weak, calls will not work.

RoamingAbroad wrote:

I had the same issue while traveling in China: not able to make or receive calls. Data and text still work. Many hours with T-Mobile support did not help. Tried all kinds of things such as reboot the phone, reset network and all settings, call #766# after manually selecting network between China Mobile and China Unicom. Finally figured it out after checking web forums and got some hint from my old working company phone on Verizon without 5G function.
 

The trick is LTE. For the calls to go through, the phone needs to enable VoLTE, and there should be strong LTE signal, and of course roaming needs to be on. If any one of the above conditions is not met, then the call cannot be completed. 
 

The reason for this is that calls may default to 3G but 3G service is being phased out. And if you set to 5G, if will not work b/c T-Mobile's roaming agreement with Chinese carriers do not cover 5G. So if one does not force VoLTE on the phone or LTE signal is absent or weak, calls will not work.

 

Update: played around a bit more and further details are uncovered. I used the iPhone Field Test Mode to determine the signal strength under different conditions/locations (by calling *3001#12345#*) and found out even if the LTE signal is strong around -80 dBm, calls still  cannot go through. The T-Mobile configuration will force the phone to go out of LTE mode when making or receiving a call. Therefore outgoing and incoming calls cannot be completed. This is definitely a T-Mobile problem in China.