* {{cite journal|url=https://archive.org/details/bstj39-4-995|last1=Deininger|first1=R.L.|title=Human Factors Engineering Studies of the Design and Use of Pushbutton Telephone Sets|journal=Bell System Technical Journal|date=4 Juli 1960|volume=39|issue=4|pages=995–1012}}
* Frank Durda, [http://nemesis.lonestar.org/reference/telecom/signaling/dtmf.html Dual Tone Multi-Frequency (Touch-Tone®) Reference], 2006.
* [http://www.itu.int/rec/T-REC-Q.24/en ITU-T Recommendation Q.24 - Multifrequency push-button signal reception]
