Context. A phone column has numbers written with all kinds of separators — hyphens and spaces — and you need them as plain digit strings.
Your task. Write clean_phone_numbers(df) that returns the phone column as a Series with every hyphen (-) and space removed.
Example. "98-765 43210" → "9876543210".
Notes. Return a Series. Remove both hyphens and spaces. Values with no separators pass through unchanged.