EMER DSCNT INITIATED BY THE PIC OF AN LR25 WHEN THE CABIN STARTS A RAPID CLB. FLT CREW UNABLE TO COMMUNICATE WITH ATC WHEN OXYGEN MASK MIKES ARE MALFUNCTIONING; ON FREQ WITH ZZZ.

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WE WERE ON THE ARR TO ZZZ AT FL410. CTR CLRED US TO DSND TO FL330. I INITIATED THE DSCNT AND CALLED FOR DSCNT CHKLIST. AS THE CABIN WAS SET TO FIELD ELEVATION I NOTICED THE CABIN ALT CLBING RAPIDLY. I PUT ON MY OXYGEN MASK AND BEGAN AN EMER DSCNT. THE FO ALSO PUT ON HIS OXYGEN MASK AND BEGAN TO ATTEMPT MANUAL CTL OF THE CABIN. THE FO SQUAWKED 7700 AND ATTEMPTED CONTACT WITH THE CTR. A FAULTY OXYGEN MASK MIKE PREVENTED CONTACT. CTR HAD US ACKNOWLEDGE THEIR INSTRUCTIONS WITH SQUAWK IDENT. WE CONTINUED THE EMER DSCNT TO 16000 FT MSL AT WHICH POINT CTR CLRED US TO 11000 FT MSL. CONTACT WITH CTR WAS RE-ESTABLISHED THROUGH THE NORMAL MIKE AT THIS POINT. WE RESET THE EMER BLEED AIR AND CONTINUED TO OUR DEST USING MANUAL CTL OF THE CABIN ALT. ESTABLISHMENT OF PERIODIC FUNCTIONALITY TESTS OF THE OXYGEN MIKE SYS MAY PREVENT LOSS OF COM IN CERTAIN SITS AS THIS. SUPPLEMENTAL INFO FROM ACN 624608: THE CAPT INITIATED A NORMAL DSCNT TO 33000 FT UNTIL THE CABIN HAD CLBED FROM 6000 FT CABIN ALT TO 8000 FT CABIN ALT. AT THAT POINT WE TRIED TO BRING THE CABIN DOWN MANUALLY (IN AUTOMATIC MODE) WITH THE CHERRY PICKER. THE PRESSURIZATION WOULD NOT RESPOND AND THE CAPT CALLED FOR AN EMER DSCNT CHKLIST. ONCE THE CHKLIST GOT US TO A POINT THAT WE HAD THE CABIN UNDER CTL; THE PF STAYED ON OXYGEN AND THE PNF TOOK OFF HIS OXYGEN MASK TO ESTABLISH EFFECTIVE COM. THIS WAS AT APPROX 23000 FT MSL WITH A CABIN ALT OF APPROX 8000 FT. IT IS NOW PART OF COMPANY OPS TO CHK THE MIKE IN THE OXYGEN MASK WHEN CHKING THE AVAILABILITY OF OXYGEN FROM THE MASK.