THE FO OF AN ACR TWIN JET FAILS TO LEVEL OFF AT AN AMENDED ALT DURING AN UNEXPECTED HOLD 11 MI E OF PVD; RI.

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I WAS THE PF ON THE LEG TO BOS. WHILE PROCEEDING INBOUND ON THE NORWICH 2 ARR DSNDING TO 11000 FT; THE CAPT COMPLETED THE DSCNT CHKLIST AND LEFT FREQ TO MAKE A CABIN PA ANNOUNCEMENT. DURING THAT TIME; ZBW ISSUED HOLDING INSTRUCTIONS AT PVD. WE WERE RELATIVELY CLOSE TO PVD AND THE FMS NEEDED TO BE PROGRAMMED FOR THE HOLD. ZBW ISSUED HOLDING AT; WHAT I READ BACK AS; 11000 FT. AS WE PASSED THROUGH 17000 FT; THE CAPT RETURNED TO THE FREQ. AS WE APCHED 16300 FT; ZBW ASKED OUR ALT. I READ BACK 'THROUGH 16300 FT.' ZBW THEN REPLIED WITH THE ASSIGNED ALT OF 17000 FT. I STATED I READ BACK 11000 FT AND PROCEEDED TO CLB BACK TO 17000 FT. THE CAPT ASKED ME WHAT WAS HAPPENING. I TOLD HIM THAT THE LAST KNOWN ASSIGNED ALT WAS 11000 FT; AND THAT I ALSO RECALLED ACKNOWLEDGING 11000 FT DURING THE READBACK WITH NO QUESTIONS FROM THE CTR. TO THIS WRITING; HOWEVER; I'M NOT SURE IF I MAY HAVE MISSED A 17000 FT ASSIGNMENT SINCE I WAS PREOCCUPIED WITH SETTING UP THE HOLD ON THE FMS. I BELIEVE THAT HAD THE CAPT BEEN IN THE LOOP; VERSUS MAKING A PA ANNOUNCEMENT; THAT THIS INCIDENT WOULD NOT HAVE HAPPENED. I ALSO FIRMLY BELIEVE THAT ALL NON ESSENTIAL PA ANNOUNCEMENTS SHOULD BE RESERVED FOR ABOVE FL180. THE NE CORRIDOR IS TOO BUSY OF AN AREA TO HAVE STERILE COCKPIT PROCS NO LESS THAN BELOW FL180. AS THIS TESTIMONIAL SHOWS; 1 PERSON OUT OF THE LOOP IN A HIGH DENSITY ENVIRONMENT CAN PROVE TO BE A POTENTIAL DISASTER WAITING TO HAPPEN. I CAN ASSURE YOU THAT I WILL RE-EVALUATE MY PROCS AND THAT I WILL PRACTICE 'STERILE COCKPIT' ANY TIME BELOW FL180 IN HIGH DENSITY TFC AREAS.