Mont. Code Ann. § 7-13-4314

Any person, firm, or corporation receiving water or sewer service outside of incorporated city limits may be required by the city or town, as a condition to initiate such service, to consent to annexation of the tract of property served by the city or town. The consent to annexation is limited to that tract or parcel or portion of tract or parcel that is clearly and immediately, and not potentially, being serviced by the water or sewer service.