Never heard of AvCall tones, but based on your description, it sounds identical to Quik-Call 1 (2+2) tones, like the tones used in the TV show EMERGENCY!
and your station's siren tone probably sounds like this
The fire tone-out feature of the Uniden DMA scanners will not decode these codes, because the encoder essentially "plays" the two mixed tones at the same time. Simply choosing one tone or taking the average of the two tones will not produce a usable tone value.
I'd love to see the decoder your fire department uses to receive these tones!