(a) The territory of a district consists of:
(1) the territory of the county for which the district is established; and
(2) for each municipality partially located in the county for which the district is established, the territory of that municipality located in another county.
(b) If a municipality that is part of a district annexes territory that is not part of the district, the annexed territory becomes part of the district.
(c) A public agency located wholly or partly in a county adjoining the county for which the district is created and that has received 9-1-1 service through a regional planning commission interlocal agreement with the district for at least 10 years may become part of the district by resolution of the agency's governing body and approval by the board.