Replace the R253 (a resistor) with a 0.1uf capacitor to get the sidetone going properly.
The long story:
I was fooling around with an upgrade to Raduino, (more over the weekend) . I noticed that the sidetone was not proper. After a day's frustration, I put a scope probe on the sidetone line and saw that as the sidetone made a square wave of 0 to +5v and back, the average voltage rose to 2.5 v. This was directly coupled (well, me!) to the LM386 audio amp's volume control. Replacing the final resistor(R253) in the low pass filter with a 0.1uf capacitor fixes the problem.
The problem was that the sidetone would 'vanish' after a start blip. That's because the LM386 was being pushed to the rail by the 2.5v DC on the input!
I am enclosing a picture of the mod.