Every telephone company organized before May 23, 1985, under special or general law, for the transaction of a telephone exchange business, in whole or in part, is limited in its operation, so far as pertains to the telephone exchange business, to the limits of the town or towns in which the plant and structures of such company, association or corporation actually existed and were in operation, in whole or in part, on such date, except upon a finding that public convenience and necessity require an extension of such limits as hereinafter provided.