PLT OF GA SMA ACFT INADVERTENTLY PENETRATED AN ARSA AIRSPACE PRIOR TO RECEIVING CLRNC.

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VFR FLT FROM HMO; MEXICO; TO EL PASO; TX. AT 7500 FT; 50 NM FROM JUAREZ; CONTACTED JUAREZ APCH. IMMEDIATELY TOLD TO CONTACT EL PASO APCH. ATTEMPTED TO DO SO. WAS TOLD RADIO GARBLED. I REMAINED CLR OF ARSA; MADE NUMEROUS ATTEMPTS TO CONTACT EL PASO APCH FSS (TO GIVE 15 MINS PRIOR NOTICE FOR BORDER XING). CLBED TO 10000 FT. FINALLY WAS ABLE TO COMMUNICATE (WITH MUCH DIFFICULTY) WITH FSS. FSS VERIFIED FLT PLAN INFO; DEST; ETC. GAVE ME SQUAWK CODE. I ASSUMED; SINCE I WAS GIVEN SQUAWK CODE; THAT FSS WAS COORDINATING MY ARR WITH APCH; IN LIGHT OF MY RADIO PROB. I ENTERED ARSA; CONTACTED EL PASO TWR. TWR IMMEDIATELY TOLD ME TO CONTACT APCH. I DID SO. APCH IMMEDIATELY TOLD ME TO CONTACT TRW. I DID SO AND IMMEDIATELY WAS CLRED TO LAND; BUT ADVISED THAT I SHOULD HAVE CONTACTED APCH BEFORE ENTERING ARSA/ATA. (I VERY BRIEFLY APOLOGIZED SAID I THOUGHT I HAD!) AFTER LNDG; CLRING CUSTOMS; I VOLUNTARILY PHONED APCH TO DISCUSS SIT. BEFORE LEAVING ELP I HAD RADIO CHKED BY AVIONICS SHOP. DETERMINED MICROPHONE WAS DEFECTIVE; PURCHASED NEW MICROPHONE.