strip_country_code() returns msisdn without country code.
write_to_file() appends new line internally instead of requiring `info` to come with a new line.
is_valid() returns True if response is valid. Logs invalid response in error log.
Write msisdns with valid responses into a 'processed' file. This is input for mark_done.sh so we can mark only those that are done.
Provide msisdn as part of the console message for numbers without CL.