How to measure telephone DTMF tones with an oscilloscope?
DTMF tones consist of a Row and Column tone. On many handsets, pressing two of the row buttons simultaneously will generate only the row tone, and pressing two of the column buttons will generate only the column tone.

The columns are assigned to the high frequency group and the rows are assigned to the low frequency group.

The matrix in Hertz can be found at:
this Wikipedia page.

In addition, DTMF tones exhibit Twist, which is a difference in amplitude between the row and column tones. If this difference is incorrect, the receiver may not interpret the tones properly.

The high frequency tones are transmitted at a nominal -6dB relative to the low frequency group; i.e. the voltage of the high frequency tones is 50% of the voltage of the low frequency tones.

HTH