1) It's spelled "digital". There are excellent spell-check programs available for most browsers.
2) Dual-Tone Multi-Frequency (DTMF) is NOT digital. DTMF is comprised of two analog tones, superimposed over each other.
In theory, any encryption can be cracked if you have a have some part of the message that is known, or you can reasonably guessed to be in the message. However, there are practical limits to this.
A scanner would be pretty impractical for this application, however, as would using a computer. That would decidedly cause a delay and would not be real time. There are at least two ways to do this cheaper and easier, and in real time.
Of course, this begs the question: "What part of 'illegal wiretap' don't you understand?"