No back-space on iPhone keypad. How do I get it back?

See the little x to the right of the green send button. That's the delete key. It will only appear once you have input a number.

When you bring up the keypad to enter a phone number, once you start entering numbers, a little x will appear next to the bottom green circle with phone icon in it. Tap the little x to delete numbers you entered.

The backspace key should appear to the right of the green send button once you start typing. If you're using dark mode, it can be a bit difficult to see the key outline but just tap on the "x".