I'm new to digital signal processing, and I've been looking into FFT, filtering, Matlab etc. What I'd like is to build a program which will detect when a certain BEEP is transmitted through a trunked voice channel (it signifies an impending critical message) being received via SDR. Through FFT I have ascertained it consists of three frequencies (500Hz, 600Hz and 700Hz). In essence -- somewhat like a phone call with DTMF -- I'd like to monitor audio for the presence of this beeeeep tone and have the software recognize when this is occurring.
My concern is that voice transmissions will cause these three frequencies to peak, making it difficult to determine whether it is due to the alert beep or somebody speaking.
I believe this might be accomplished through some means of measuring the length of the beeps, however this may be a misunderstanding through my inexperience in the field.
I'd appreciate any advice, and if my question isn't directly answerable, please push me in the right direction.