A local exchange company may not charge a residential customer or single line business for basic local exchange service based on the duration of a call or on the time of day that a call is made; provided, however, that such restriction shall not apply in any case where a customer or business requests charges based on the duration of a call or on the time of day that a call is made. This Code section does not prohibit a local exchange company from offering discounts based on the time of day that a call is made if the company also offers basic local exchange service at a rate permitted under Code Section 46-5-166.