Assuming this a 6900 based board, the 2517 R Beep Oscillator is Q40, in the front right of the board (parts side up). R193 is what feeds the Beep tone into the Mic Preamp circuit. It is a 1.5 Meg (Brown/Green/Green) resistor, you could replace it with a higher value, like 4.7 Meg (Yellow/Purple/Green) or a 10 Meg (Brown/Black/Blue). This should tamper down the loudness of the R Beep.
As to changing the tone frequency, C161 (.015uF or marked 153) , C162 and C163 both are .0047uF (marked 472). Changing C162 and C163 to higher values, like using the same value 153 used in C161 will lower the frequency of the tone. Green "Chicklet" Mylar Capacitors are less temperature sensitive than the round Ceramic caps usually used here.
This is how they change the Roger Beep to a Roger Poot, by changing all 3 caps to way higher values. I can't remember what I used to use (over 20 years ago, now), but that is what is done. If you take the frequency down a little too far (Poot), you might want to Decrease R193 to something like 100k, as the lower the frequency you take it to, the less loud it becomes.
If you want a longer or shorter Beep duration, change C159 (2.2uF electrolytic, next to J10) to a larger or smaller value.
