Have you tried the beta "sonic lab" fax service?
Not sure it will support outgoing international faxes.
Do voice calls to south korea work from the same phone line?
What do you mean by you can't send?
Does it ring? Do you hear the fax machine picking up and trying to do handshake?
What does the fax machine report read?
It could be you have disabled international calls on your voice line, so you could not do voice calls either.
It could be that sonic uses some voip along the way that is not good enough for fax. There may be ways to use a different voip codec.
It could be that fax in south korea use a different standard (unlikely, i have not researched it)
So basically, you will need to give us more details than "it does not work" so we can guess a plausible reason it does not work.