A DHC-8 FLT CREW EXPERIENCED DIFFICULTY WITH THEIR SELCAL SYSTEM WHICH PREVENTED THEM FROM RECEIVING A MESSAGE FROM DISPATCH REGARDING A MISSED FREQUENCY CHANGE.

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THIS WAS A ROUTINE FLT TO ZZZ. WHILE CRUISING AT FL200; WE RECEIVED A SELCAL ALERT INDICATING THAT OUR DISPATCH DEPT WAS ATTEMPTING TO CONTACT US. THE CAPT; AS PNF; ATTEMPTED TO REPLY TO THE SELCAL; BUT TO NO AVAIL. HE THEN ATTEMPTED TO USE THE SYS TO CONTACT DISPATCH; WHILE ALSO DID NOT WORK. THIS WAS NOT SURPRISING TO EITHER OF US; AS THE SYS HARDLY EVER WORKS; AND HAS BEEN A PROB FOR YRS; WHICH IS A FACT KNOWN TO BOTH THE COMPANY AND THE FAA. AS NEITHER THE CAPT NOR MYSELF COULD THINK OF ANY IMPORTANT REASON THAT DISPATCH WOULD BE TRYING TO CONTACT US; WE ELECTED TO SIMPLY GIVE THEM A CALL UPON REACHING ZZZ TO SEE IF THEY WANTED US FOR ANYTHING; OR IF THE SELCAL ALERT WE RECEIVED WAS SIMPLY A GLITCH (WHICH ALSO SOMETIMES OCCURS). THE FLT CONTINUED UNEVENTFULLY FOR ABOUT ANOTHER 15 MINS; AT WHICH POINT THE CTR CTLR WHOSE FREQ WE WERE MONITORING CONTACTED US. WE RESPONDED; AND THE CTLR SAID THAT HE HAD BEEN TRYING TO GET IN TOUCH WITH US FOR 60 MI. THIS CAME AS QUITE A SURPRISE TO US; SINCE WE HAD BEEN ABLE TO HEAR THE CTLR TALKING TO OTHER AIRPLANES ON A REGULAR BASIS DURING THIS ENTIRE PERIOD; AND HAD NEVER ONCE HEARD HIM ATTEMPT TO CONTACT US. WE INDICATED TO THE CTLR THAT WE HAD BEEN ON THE FREQ AND HAD NOT HEARD HIM CALLING; IF IN FACT HE HAD BEEN TRYING TO REACH US. AT THAT POINT HE HANDED US OFF TO THE NEXT SECTOR; AND NOTHING ELSE WAS SAID. AFTER LNDG; WE CALLED DISPATCH AND LEARNED THAT ATC CONTACTED OUR COMPANY DISPATCH AFTER BEING UNABLE TO REACH US ON THE RADIO; AND THAT IS THE REASON WE RECEIVED THE SELCAL ALERT. WE NEVER DETERMINED WHY ATC WAS NOT ABLE TO REACH US ON THE RADIO. IT IS MY OPINION THAT THEY WERE TRYING TO CONTACT US ON A DIFFERENT FREQ FOR SOME REASON. I DO NOT BELIEVE THAT WE MISSED A HDOF AT ANY TIME; OR MISSED ANY RADIO CALLS INTENDED FOR US. LUCKILY; NO AIRSPACE CONFLICTS OR LOSS OF SEPARATION OCCURRED AS A RESULT OF THIS INCIDENT. HOWEVER; IT HIGHLIGHTS ONCE AGAIN THE UNSAFE SITUATION CREATED BY THE DEFECTIVE SELCAL SYS IN OUR AIRPLANES. IF THIS HAD BEEN A CRITICAL SITUATION; OUR DISPATCHER WOULD HAVE HAD NO WAY TO COMMUNICATE WITH US AT ALL.