AIR AMBULANCE HELI PLT FLEW THROUGH CLASS D AIRSPACE WITHOUT CLRNC DUE TO NOT BEING ANSWERED AFTER SEVERAL ATTEMPTS.

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RETURNING TO BASE FROM A PATIENT FLT. CHKED WX BEFORE DEP. WX RPTING MVFR; 1200 FT SCATTERED; 1500 FT BROKEN; BETTER THAN 5 MI VISIBILITY. LEAVING EASTWARD; CALL-UP TO ZZZ TWR WAS INITIATED WITH NO RESPONSE (THE SAME SIT HAS BEEN RPTED TO ME BY MY FELLOW PLTS). SEVERAL SUBSEQUENT ATTEMPTS WERE MADE; AGAIN WITH NO REPLY FROM TWR. THINKING MAYBE THAT THE TWR HAD CLOSED; I CONTINUED TRANSITIONING THE AREA. FINALLY; AS I WAS ABOUT TO EXIT THE AREA; THE CTLR CALLED UP AND ASKED 'N-NUMBER SO AND SO; IS THAT YOU?' I REPLIED 'YES; I'VE BEEN TRYING TO CALL YOU.' THEN THE CTLR SAID 'JUST BECAUSE YOU CAN'T REACH US ON THE NORMAL FREQ; DOESN'T MEAN YOU SHOULD CONTINUE THROUGH THE AIRSPACE.' 'WE COULD HAVE OTHER ACFT OUT THERE.' 'YOU COULD HAVE TRIED 121.5.' I AGREED; I COULD HAVE CALLED ON THE EMER FREQ. I DON'T WANT TO POINT FINGERS; BUT THERE SEEMS TO BE PERIODS WHEN THE CTLR IS NOT BEING ATTENTIVE. I BELIEVE THAT THE RADIO IN A 24 HR FACILITY SHOULD BE MONITORED 24 HRS. THIS WOULD PREVENT FUTURE RECURRENCE OF THE SIT.