I am unable to setup UPI for USA number on my iPhone

For what it is worth, here's your bank's FAQ on the iMobile Pay app.

https://www.icicibank.com/mobile-banking/imobile-faqs

The FAQ says, in part, that

• Your mobile phone number must already be registered with the bank.
• You must have mobile (cellular) data enabled – the app will not activate over Wi-Fi.
• There may be a charge for the SMS message used to activate the app.

The issue may be that your mobile phone number is not a domestic phone number for the country of India. In that case, I don't know whether there would be a legal issue that would prevent you from what you want to do at all - or if you would need extra assistance from ICICI's customer service to get the situation straightened out.

https://www.quora.com/How-do-I-activate-ICICI-bank-Android-app-when-I-am-outside-India-and-my-registered-mobile-number-is-not-active

How would this be an Apple issue?

Phone service on your iPhone is handled by your carrier, not Apple.

Same issue. ICICI customer service has been totally frustrating. Takes two days and tells me it is T-Mobile (phone service carrier issue). It was working couple months back and now last two weeks app is asking me to re register my mobile number and keeps failing. it has even happened with OTP. I was not getting OTP and their response was same - carrier issue. Every time they update app they break stuff. Wonder who tests these apps and approves for production release - knowing they have clients around the world.

i am pretty much done. Taking my business else where. I really hope they learn customer service soon.