ARTCC controller reported having control for an aircraft from another facility but the frequency used didn't provide coverage at the aircraft's position and altitude while the aircraft was on a heading and altitude that would be below the MIA. Communications was established and the aircrew reported VMC and was issued a climb to the MIA.

Narrative

Aircraft X and another aircraft handing off to me inside CPR approach airspace during a relatively higher-than-usual volume and complexity situation. Accepted those handoffs shortly after they began flashing. Aircraft X mode C read 110. As I recall; just before I accepted the hand off for these aircraft; CPR called on the hotline. I answered the call and the Controller advised that these two aircraft were my control. I acknowledged and accepted RADAR contact. Then; I got back to work on other aircraft throughout the sector. By offering control of the aircraft; I assumed I would have the opportunity to talk to these aircraft in CPR airspace; otherwise the control for these aircraft would be pointless. By accepting control well before the aircraft reached the boundary of my airspace; I ensured I would be able to issue restrictions that are needed for the aircraft to enter my sector safely. The MIA directly adjacent to and overlapping CPR approach airspace by two miles; is an MIA of 120. Aircraft X had a route of flight that cut through this MIA. There had been weather deviations (and was a CPR arrival near these two that was deviating at the time) throughout the morning within my sector and I considered control for these aircraft to involve deviations for weather. I awaited a call from Aircraft X and that call did not come within a time that would allow me to turn the aircraft or climb it for the MIA of 120. I noticed the conflict with the MIA as the MSAW began to flash. I immediately reached out to the aircraft with a low altitude alert and the aircraft did not respond. I called CPR and asked them to climb the aircraft or ship it to me so that I could utilize the control I was given. It was busy; and had it not been; I may have deduced that CPR had put the aircraft on a frequency that would not allow me to communicate until the aircraft was outside of CPR airspace. I have been told (simply through word of mouth) by a previous CPR controller; that the staff at CPR is aware that there are issues with the coverage on 132.1 (the frequency CPR had issued when switching the aircraft to me) reference the high elevation in the MIA of 120. On a subsequent call to CPR; another Controller there told me that they had shipped Aircraft X shortly after the call offering me control. I asked them to try to get ahold of the aircraft; I attempted to reach the aircraft on guard frequency; I issued multiple low altitude alert warnings; and alerted the CIC as soon as I was able. Eventually; Aircraft X answered my call on 132.1; read back the climb to 120; and advised that conditions were VFR and that the pilot had good visual on terrain. I followed up with CPR; advising to consider the frequency limitations of 132.1 and to consider the higher MIA that reaches into their airspace. Under a given altitude and within the boundaries of CPR approach; aircraft should probably be given 135.6 to communicate with ZDV. 135.6 has a transmitter receiver site at CPR and 132.1 is far south at CKW and MBW. The aircraft was too low and too far away from these site to hear my call.