I want to download the FamPay app on my iPhone

If you're talking about this app:--> https://apps.apple.com/in/app/fampay-upi-payments-for-teens/id1499806454, it requires iOS 14 or later.

Check the current iOS version running on your unidentified iPhone model. If you need to update it, these are the instructions:--> Update your iPhone, iPad, or iPod touch, but before proceeding, check your device compatibility:

Also, if your iPhone is running iOS 14 or later and get an error message when trying to download: See here -> If you can’t download or update apps on your iPhone or iPad – Apple Support (UK)

If the issue persists: https://getsupport.apple.com/topics