upi doubts

Yes outgoing SMS is mandatory to register for UPI. 

You can opt for any reputable UPI app mentioned earlier.

https://www.npci.org.in/what-we-do/99/product-o...

Using *99# USSD Code, UPI Money Transfer is Possible. No recharge or Outgoing SMS is required.

I link UPI to every feasible app. Cred and Amazon (funky in terms of merchant payments in my experience ) are the best by far. Don't think been ever been asked to reverify unnecessarily. Very lenient even if SIM slot changed or SIM removed in favor of a microSD. Just restore the regd. SIM to the original slot and it's all good. Paytm is kinda OK. Some like NSDL, Payzaap are not that good as they seem to mandate re-verification after almost every app update.