how to show numbers and letters on iphone keyboard at the same time

I have an iPhone 15 but how do I tap in numbers & letters on the phone when asked by caller

It's simple.Download the Microsoft SwiftKey et voila!

None of the various iPhone models in my family (5) can do that.

The iPad, however, does.

Meanwhile, this is a neat trick from https://www.iphonelife.com/content/how-to-type-numbers-stay-standard-iphone-keyboard:

How to access numbers and symbols without leaving the ABC keyboard on your iPhone.

To begin, open an app where you use your iPhone's keyboard, such as Messages or Notes. 

1. Tap the text field to reveal the standard iPhone keyboard, and begin typing.
2. Touch and hold the 123 button. It works best to do this with your non-dominant hand, as you'll need to keep holding it while you type the numbers and/or symbols.
3. Type the numbers or symbols you need using your other hand, then release the 123 button.

william98 wrote:

my wife swears her iphone 6s keyboard used to do that but it mysteriously disappeared.

Not from Apple. The app store has some, e.g

https://apps.apple.com/app/typefinity/id1512009869