AN EMS PLT ATTEMPTED TO CALL XXX TWR ON YYY TWR'S FREQ. THE RADIO HAD BEEN KNOWN TO SWITCH FROM ONE TUNING HEAD TO THE OTHER WITHOUT PLT ACTION. THE PLT MAY HAVE BEEN IN XXX'S AIRSPACE BY THE TIME HE CALLED.

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NORMALLY FLY HELI #1. HELI #2 IS USED AS STANDBY WHEN HELI #1 IS IN MAINT. I QUALIFIED IN HELI #2 IN AUGUST AND HAD NOT FLOWN ONE SINCE. I TUNED BOTH X AND Y TWR FREQS IN THE #1 RADIO SINCE THE EMS MISSION WOULD TAKE ME THROUGH BOTH AIRSPACES. RADIO #2 WAS USED FOR COM WITH BASE. PRIOR TO LIFTING I SELECTED X AS THE ACTIVE FREQ. AS I CLRED THE HOSPITAL BUILDINGS; I CALLED X TWR FOR A TRANSITION TO THE NW AT 1200 FT. A VOICE ANSWERED 'THIS IS THE TWR SAY REQUEST.' I GAVE A POS RPT AND WAS TOLD TO SQUAWK XXXX. TWR STATED THAT THEY WERE NOT PICKING ME UP ON RADAR. I STATED THAT I WAS ABOUT 5 MI NE OF THE FIELD. AFTER I MADE A FEW OTHER VERBAL EXCHANGES ATTEMPTING TO BE PICKED UP ON RADAR; I INCLUDED 'X' IN MY POS RPT. THEN THE TWR INDENTED ITSELF AS Y AND TOLD ME TO CONTACT X. I WAS SURPRISED TO SEE Y'S FREQ IN THE ACTIVE WINDOW OF THE RADIO. I CONTACTED X WHICH HAD ME ON RADAR AND CLRED ME FOR THE REQUESTED TRANSITION. THE REST OF THE MISSION WENT SMOOTHLY. ONE OF OUR OTHER PLTS SAID THAT THIS RADIO HAS HAD A HISTORY OF DESELECTING THE ACTIVE FREQ. I SHOULD HAVE CHKED BEFORE XMITTING. THE RADIO WILL BE SERVICED. THE DOCTOR AND NURSE ON BOARD WERE LISTENING TO THE RADIO XMISSIONS AND ALSO THOUGHT I WAS TALKING TO X. TWR CTLRS SHOULD IDENT THEMSELVES ON INITIAL CALL. PLT SHOULD BE MORE SPECIFIC IN POS RPTS (WHICH FIELD -- NAME IT). I MAY HAVE BEEN WITHIN X'S AIRSPACE THOUGH THEY DID NOT SAY WHERE THEY HAD ME ON RADAR.