Add a string prefix to each value in a string column using Pandas if length is x

Question

I'm trying to add a string prefix to a telephone no if it's missing i.e. the len() is 8

def f(x):
    if state_df['Work Phone'].str.len() == 8:
        state_df['Work Phone'] = '02' + state_df['Work Phone']

However this has no effect on the state_df