Oracle trim whitespace on the inside of a string

You want to try replace (telno, ' ', '').

a better approach is to use a regular expression to remove all spaces within a string

SQL> with t as (
    select 'AAA    BBB CCC' col from dual union
    select 'DDDD EEE     F' col from dual 
    )
    --
    -- actual query:
    --
    select regexp_replace(col, '[[:space:]]+', chr(32)) col
    from   t;

COL

AAA BBB CCC DDDD EEE F