A320 CREW FLEW OUT OF RADIO RANGE. THE CREW GOT A TCASII RA IN ZOB CLASS A.

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I WAS FLYING IN CRUISE FL290. I NOTICED THAT WE HAD NOT TALED TO ATC LATELY AND THE RADIO XMISSIONS WERE WEA. I ASED THE FO TO CH IN WITH ATC. HE DID AND WAS UNABLE TO GET THEM UP. WE WROTE AN ACARS NOTE TO DISPATCH ASING THEM TO GET US A FREQ. THEN WE GOT A TCASII RA TO CLB. WE CLBED UP TO FL300; THEN GOT A CLR OF CONFLICT MESSAGE. WE DSNDED BAC DOWN TO FL290; CALLED ON 121.5 TO SEE IF SOMEONE COULD HELP US WITH AN ATC FREQ. AN AIRLINE PLT CAME BAC WITH A FREQ WHICH WE CONTACTED ATC. WE RPTED THE TCASII AND THEY SAID THEY WERE AWARE OF IT. WE CONTINUED ON WITH AN UNEVENTFUL FLT. WE DO NOT NOW IF WE MISSED AN ATC CALL; OR IF THEY FORGOT TO HAND US OFF. WE WERE NEVER OFF COURSE OR ALT WHEN WE GOT THE TCASII. WE BOTH WERE TIRED FROM THE EARLY DEP OUT OF SFO AND DELAYS IN PHX. DISPATCH FINALLY GOT BAC TO US WITH THE SAME ATC FREQ THE AIRLINE PLT HAD GIVEN US. WE WERE ALREADY IN CONTACT BY THEN. WE LANDED UNEVENTFULLY IN LGA. SUPPLEMENTAL INFO FROM ACN 626183: WE WERE ON COURSE; ON ALT; WHEN WE REALIZED THAT WE HAD NOT HEARD FROM ATC FOR AWHILE; SO WE CALLED THEM AND GOT NO ANSWER. APPARENTLY; WE HAD FLOWN OUT OF THEIR SECTOR. WE DO NOT THIN THAT WE MISSED A CALL; WE THIN THEY SIMPLY DID NOT HAND US OVER. WE NEVER DID SEE THE ACFT; IF THERE WAS ONE; THAT CAUSED THE TCASII WARNING AND RA. THE CLOSEST AIRPLANE THAT I SAW ON THE RADAR WAS AT OUR ALT ABOUT 30 MI AWAY; IN FRONT OF US. WE SUSPECT IT WAS MAYBE A FIGHTER SENT TO TAE A LOO?