Academic Papers - Dual-tone multi-frequency signaling

Hi Freelancers. I'm looking for a paper on Dual-tone multi-frequency signaling. I'm looking for definitive resources on the above with a much more comprehensive look then you would get from a Wikipedia page. You can discuss technology, evolution, history, current state, applications, etc. I don't like to cite word cound as it should be as long to create a comprehensive product but 4000-6000 should be appropriate. Please respond with outline, past work, and credentials. Thanks!