S.D. Codified Laws § 49-31-86
Prices for residential and business local exchange service, both recurring and nonrecurring, for a telecommunications company with more than two hundred thousand retail access lines in the state may not be changed unless reduced by the company. If the telecommunications company reduces its prices for residential or business local exchange service pursuant to §§ 49-31-1 to 49-31-88, inclusive, it may not subsequently increase such prices. However, this restriction does not apply to promotions that last ninety days or less. The telecommunications company shall publicly announce the beginning and ending date of the promotion period. The provisions of § 49-31-4 and §§ 49-31-12 to 49-31-12.5, inclusive, do not apply to prices for services regulated by this section.
SDCL 49-31-86