In a message posted above, Masato says the earliest known recordings were from 1937. I'm guessing that a song based on telephone service, can't be earlier than the 1920s - so it quite likely was composed during the period that is not in the public domain.
Several entries in the Harry Fox Agency songfile.com Website say the song is "traditional" or public domain. I think I'd feel safe saying the song is in the public domain.
-Joe-