Filtering out unwanted square wave (Radio: DCS/DPL signal)
If you know that it is a square wave, and the frequency is roughly constant, I would think about synthesizing an inverted copy of it and adding it to the signal.  The remainder would be your speech, plus some residual harmonics of the square wave frequency, which might be dealt with by a 1-channel adaptive filter (see Sec. IIC of this link: http://www.cs.cmu.edu/~aarti/pubs/ANC.pdf).  It's possible that the adaptive filter will do it all by itself--I have used them with good results on periodic interfering signals with attention to the delay setting involved.
In order to get the parameters of the inverted signal, you would do one of two things:
1. Fit a two-parameter square wave to the signal using least squares minimization (two parameters being the initial time delay and the amplitude, or add other parameters as you see fit).
2. Use an FFT to estimate phase and amplitude of the fundamental sine wave, then use the theoretical ratio of the amplitude of a square wave to its fundamental (isn't it 2/pi?) to reconstruct the square wave from that.

Which of these two approaches you pick depends on the bandwidth of the speech and its amplitude relative to your DCS signal.  Good Luck!