Every telephone company which offers dial tone line or substantially equivalent local service shall offer to its residential and business customers at least one offering of such service consisting of a single dial tone line, including associated usage, for the purpose of two-way voice communications within a local calling area at a flat rate unless there was no telephone company offering such a class of service at a flat rate in the local calling area on January 1, 1979. No residential or business customer shall be forced to accept local measured rate service for calling within a local calling area based on the number of calls, length of call, distance or time of day, not in effect on January 1, 1979. Nothing contained herein shall prohibit the Commission from approving the voluntary tariff of any telephone company based on the number of calls, length of call, distance or time of day.