Granted Patent
Utility
US 5,835,574 · App. 08/766,097
Dual-tone multi-frequency signal transfer protocol
Inventor:
Chi K. Lam
Patented Case
View Patent ↗
Granted: Nov 10, 1998
Filed: Dec 16, 1996
Inventor
Chi K. Lam
Assignee (at issue)
AT&T CORP.
This patent predates the USPTO's electronic file wrapper — prosecution documents from this era exist only in the Patent Office's paper records. A certified copy of the file history can be ordered from the USPTO's paper archives.