Area code 975 was scheduled to enter service in late 2001, but its activation was subsequently postponed indefinitely after the reevaluation of the need for a relief plan for Numbering Plan Area 816, covering the western portion of Missouri, with Kansas City, Independence, Lee’s Summit and Saint Joseph among the main locations served.
This year (2021) in August the Missouri Public Service Commission announced a new effective date for the introduction of the new area code 975, now scheduled to enter service on February 10, 2023, overlaying existing 816 area code and basically covering the exact same geographic area.
The dialing plan for the newly formed overlay complex 816/975 will be: 10 digits for all local calls and 1+10 digits for all toll calls (telephone service providers may also allow 1+10 digit dialing for local calls). This dialing plan is already in place for NPA 816 since October 24, 2021.
See official announcement published by the NANPA (PDF document)