Hi.
You tried to measure the frequencies by tapping the wire? Which wire?
From the keypad? from the telephone line?
You cannot probe the telephone line, you must use a transformer in SERIES with ONE of the telephone line wires to be able to extract or sample the audio or tones.
A frequency counter will read erratic anyway, as DTMF is Dual Tone... two simoultaneous tones as listed above. Only pressing two numbers at the same time will output a single tone.
Miguel