Commercial Fixed Wing
Air Traffic Control Fully Certified
Communication Breakdown
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Landing Without Clearance
General None Reported / Taken
Human Factors
An Air Carrier was cleared for a LOC Runway 2 approach to the Glacier Park International Airport; subsequently; RADAR service was terminated and the Air Carrier was advised to contact Glacier Tower on 124.55. Two minutes later; the Air Carrier came back on Center frequency and said Tower did not respond. I attempted to contact Tower on the shout line with no success. The Area Supervisor was notified and he attempted to call the Tower via the telephone line with no success. The Air Carrier landed on Runway 2 without clearance from Tower. The Air Carrier cancelled IFR on Center frequency. Five minutes later Tower called on the shout line and said the Air Carrier was an arrival and when I asked if everything was okay; he said everything was fine.
ZLC Controller described a failed communications attempt with GPI Tower by an IFR inbound on the LOC Approach.  The reporter was unable to explain the failed communications.