> > You cannot filter the DC jump out of the signal, because the jump
> has wide
> > spectrum including DTMF frequencies band.

Choosing the smallest value of coupling capacitor that will still pass
the tones can help a lot.  This forms a high-pass filter so the "thumps"
become more like "clicks".  Most devices that receive DTMF will not be
affected by such clicks.  Clamping the other side of the capacitor to the
ground (or whatever DC bias is normally present) before stopping the tone
would remove the disturbance completely.
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