How can NRIs use UPI in India?
The only way NRIs can have access to UPI is to either own an NRO account or NRE account and link it to an Indian phone number. NRO accounts would allow you to both send and receive money but NRE account only allows to send money. In addition to the hassles of owning these accounts you will also have to spend money on keeping and Indian phone number active. It is upto you to decide whether this trouble is worth it given how terrible are Indian government's KYC policies for these banks accounts.
There are some services like Cheq which are meant for foreign citizens and NRIs. However they charge Rs 800 to join and then demand that you do a KYC at some remote location thousands of miles away. For example they have around 12 KYC centers for the whole country. It is neither practical nor sensible product unless you live close to those KYC centers assuming they really exist.
Kotak811 is one of the best services to open an account 100% online without any KYC as long as you have an aadhar card linked to your phone. They give you an instant debit card thats purely virtual and you can use that for all ecommerce transactions. You also get an UPI address to which you can only add money but you can not send money to others. There is a hard limit of Rs 5000 on such accounts at any given time but for emergency this is the best account.
All restrictions on this account can go away if you do a full KYC and they claim they can send a person to your home or you can do it via video but I have never been able to do that.
In my experience UPI is not worth the hassle. It is better to just use one of your american credit cards at point of sale machines. As long as those cards do not have any foreign transaction fees. You might end up paying slightly higher price due to currency conversion but it is still better than dealing with the heavy handed approach of KYC in India.
I have heard that NPCI has allowed UPI using foreign phone numbers, does that mean we can now use UPI with our US numbers ?
The answer is no. While NPCI made that claim there is not a single UPI app that allows UPI over US numbers. I have tried all the major apps and none of them allow this. So in practice you can not use UPI with US numbers.
It is nearly impossible for NRIs to have access to UPI unless they can get an NRO account. However NRO accounts have their own problems which include regular KYC. If you fail to do KYC the bank will freeze your outgoing transfers and also demand you share personal details like SSN numbers with Indian banks which aren't really known for strong privacy protections.