The function takes a string input and returns the string with any numbers it contains converted to their spelled-out form, while it removes all characters which are not digits.

[CODE]: 
```python
def approximate_function(input_string):
    number_mapping = {
        '0': 'ZERO', '1': 'ONE', '2': 'TWO', '3': 'THREE', '4': 'FOUR',
        '5': 'FIVE', '6': 'SIX', '7': 'SEVEN', '8': 'EIGHT', '9': 'NINE'
    }
    
    output_string = ''.join([number_mapping[char] if char.isdigit() else '' for char in input_string])
    return output_string
```

Let's run the approximate function a few more times to see if it consistently matches the behavior of the original function.

COMMAND: PYTHON(./temp/function.py "combination123" "5random_characters" "testing982" "00112233445566778899")
