Center trainee reported they observed a MSAW Alert for an aircraft too low for radio coverage that was above their Minimum IFR Altitude but on the boundary of another facility.

Narrative

I took over the sector from a previous controller. In the briefing the controller told me that Aircraft X had been identified by Center; and that they wouldn't be able to talk to the aircraft due to their altitude. The controller in his briefing told me that he had stated 'Radar Contact Lost' to the aircraft as well and that they needed to contact ZZZZ Center 160 miles from their destination on a specific frequency. I signed into sector XX and once Aircraft X got close to the border; the MSAW alert started to flash 92 [9200ft] for the aircraft. The aircraft was currently in an MIA box of 6000 ft.. I tried to reach out to Aircraft X to see if they would be able to climb to 10000 ft. but there was no response. I tried on guard and no response. I told my Supervisor about the situation and we tried a higher altitude aircraft to reach the pilot of Aircraft X from a different sector. We were able to relay to the pilot to contact ZZZZ Center at that time. I called ZZZZ1 Center and asked them what their MIA was where ZZZ MIA is 9200 ft.. ZZZZ1 Center said their MIA was 6700 ft. and that the aircraft at 8000 ft. was at an appropriate altitude for them. It is confusing who's MIA to go off of. The 9200 ft. MIA for ZZZ starts at border of ZZZZ1; but ZZZZ1's MIA there is 6700 ft.. ZZZZ1 had identified them; so do we go by ZZZZ1's MIA or ZZZ MIA? Please take a look into the MIA along the border going North into ZZZZ1 airspace. It needs to be clarified.