ACR LTT ACFT FLC'S RADIO TRANSMISSION REQUESTING CANCELLATION OF IFR FLT PLAN IN A REMOTE AREA WAS NOT HEARD BY ARTCC.

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WITH THE CAPT FLYING; WE HAD BEGUN A DSCNT FOR MAMMOTH LAKES ARPT LEAVING FL220 FOR 16000. AS WE CROSSED THROUGH FL180; I CALLED ZOA AND ADVISED THEM THAT WE HAD THE VALLEY IN SIGHT AND WERE CANCELLING IFR. THE RESPONSE WE HEARD WAS 'ROGER.' THIS SEEMED TO US TO BE AN UNUSUAL RESPONSE; BUT ONE WE HAD HEARD IN THE PAST. OUR AIRLINE OPERATES UP TO 4 FLTS A DAY TO MAMMOTH. BEFORE WE HAD A CHANCE TO QUESTION THE RESPONSE WE WERE ALERTED TO BOTH THE L AND R ENVIRONMENTAL FAIL ANNUNCIATORS AND THE MASTER WARNING. OUR BLEED AIR PRESSURE HAD DROPPED TO NEAR 0 AT FLT IDLE DUE TO FROZEN VALVES ON THE BLEED AIR SYS (CAUSE IS NOT KNOWN AT THIS TIME BY MYSELF). WE CONTINUED OUR DSCNT WITH ENOUGH PWR TO MAINTAIN ADEQUATE BLEED PRESSURE AND LANDED WITHOUT FURTHER PROBLEMS. AS WE DSNDED AND THE AIR ROSE ABOVE FREEZING; THE PROBLEM DISAPPEARED. WE WENT INTO THE TERMINAL AFTER UNLOADING PAX AND BAGGAGE; AND WERE TOLD THAT ZOA HAD CALLED WANTING TO KNOW IF WE HAD ARRIVED. WE CALLED CTR AND TOLD THEM WE HAD RECEIVED ACKNOWLEDGEMENT OF OUR XMISSION. THEY SAID THANK YOU AND THE CONVERSATION ENDED. ON OUR RETURN LEG WE BEGAN THINKING THAT THE REASON FOR THE UNUSUAL RESPONSE WAS THAT OUR XMISSION HAD BEEN BLOCKED BY ANOTHER ACFT IN THAT AREA AT THAT ALT. SOMETIMES OUR XMISSIONS ARE VERY WEAK AND EASILY BLOCKED WITHOUT THE USUAL FEEDBACK.