PLT OF SMT CARGO FLT HAS COM PROB WHEN ATC ALLOWS HIM TO GO OFF FREQ TO OBTAIN WX BUT RETURN ON A DIFFERENT FREQ. WHILE OFF FREQ HE ENTERS A HOT RESTR AREA. HE IS ON AN IFR FLT PLAN.

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FLYING AT CRUISE ON AN IFR FLT PLAN; WE INITIATED COM WITH THE CTLR; PRIMARILY AS A RADIO CHK BECAUSE IT HAD BEEN SO QUIET. IT TOOK 2 OR 3 ATTEMPTS FOR THE CTLR TO ANSWER. WE WERE ABOUT TO CHK THE WX; BUT WERE GOING TO WAIT UNTIL WE HAD CHANGED FREQS; ASSUMING IT SHOULD BE AT ANY TIME. AFTER A FEW MORE MINS; WE WENT AHEAD AND ASKED PERMISSION TO DROP OFF THE FREQ TO GET WX FROM FLT WATCH. THE CTLR GRANTED PERMISSION; BUT WANTED US TO RPT BACK ON A DIFFERENT FREQ BUT HE DIDN'T KNOW WHICH FREQ. HE TOLD US TO STAND BY AND THEN CAME BACK WITH THE NEW FREQ. WE BRIEFLY SPOKE WITH FLT WATCH ON 122.0 AND THEN CAME UP ON THE NEW FREQ. THE CTLR ADVISED US THAT WE HAD JUST FLOWN THROUGH AN ACTIVE RESTR AREA. HE INDICATED THAT THEY HAD ATTEMPTED CONTACT WITH US ON SEVERAL FREQS AND THAT WE HAD MISSED A FREQ CHANGE. HE ADVISED US THAT THERE WAS LIVE ARTILLERY FIRE UP TO 4000 FT; BUT THAT WE HAD NOT CAUSED A PROB. HE DID STATE THAT IT WAS OUR RESPONSIBILITY TO AVOID FLYING INTO THAT AIRSPACE. WE WERE OPERATING ON AN IFR CLRNC AND WERE IN CONSTANT COM WITH ATC.