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As to the OP, connecting a scope direct to the phone lines, as discovered, means you find out the phone line has -48V on the one leg when on hook, and around -7 to -15V when the phone is off the hook and acting like a roughly 600R load.
If you want to see what is on the line with the scope you need to use 2 capacitors ( any value from around 47n to 1uF at a voltage from 160V to 400V) in series with the probe input and the probe earth . that way you only will get the Ac signals on the line, not the standing DC bias voltage.
That way when the phone is on the hook, you should see only a small amount of noise on the line, mostly AC mains induced noise from the long line to the exchange. You might also see some what looks like broadband noise from your ADSL connection, or bleedthrough from the other ADSL connections that share the trunk cable to your local exchange and the DSLAM in the street by it.
Any signal on it that looks like room noise would be from what is called an infinity bug, which will be picking up the room noise, using a sensitive microphone in the phone base. It typically is done with a module added to the phone, as the mic needs to be very sensitive, which the phone microphone and ringer is not.
Typically though the 3 letter agencies do not tap phones, they prefer other methods, like using a separate microphone placed secretly in the room, and by using the thing you all have, a mobile phone, which has a very sensitive microphone already and decent DSP, for use in the hands free mode.
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