why do I get the error message "'str' object has no attribute 'str'", 'occurred at index 0'" here

I have a dataframe with a column of phone numbers which I'd like to map to country (as a new column "phone_country"), based on their country code. I realise there's probably a Python library to do this, but right now I'd like to understand why I get the error message "'str' object has no attribute 'str'", 'occurred at index 0'"

I did try to ensure that the phone_number column was a string df['phone_number'] = df['phone_number'].apply(str)

def phone_country(df):
    if df['phone_number'].str[:1] == '7': 
        value = 'RUS'
    elif (df['phone_number'].str[:2] == '34'): 
        value = 'ESP'
    elif (df['phone_number'].str[:2] == '39'): 
        value = 'ITA'
    elif (df['phone_number'].str[:3] == '212'): 
        value = 'MAR'
    else:
        value = 'Other'
    return value

df['phone_country'] = df.apply(phone_country, axis=1)

Thanks in advance, I'm pretty new to Python.