Blue Box - Frequencies and Timings

Frequencies and Timings

Each MF tone consists of two frequencies, shown in the table on the left. Note that these are not the same as customer dialed Touch Tone, which is shown by the table on the right:

Code	700 Hz	900 Hz	1100 Hz	1300 Hz	1500 Hz	1700 Hz
Operator (blue box) dialed MF frequencies
1	X	X
2	X		X
3		X	X
4	X			X
5		X		X
6			X	X
7	X				X
8		X			X
9			X		X
0/10				X	X
11/ST3	X					X
12/ST2		X				X
KP			X			X
KP/ST2				X		X
ST					X	X

	1209 Hz	1336 Hz	1477 Hz	1633 Hz
Customer-dialed Touch-Tone (DTMF) frequencies
697 Hz	1	2	3	A
770 Hz	4	5	6	B
852 Hz	7	8	9	C
941 Hz	*	0	#	D

Normally, the tone durations are on for 60ms, with 60ms of silence between digits. The 'KP' and 'KP2' tones are sent for 100ms. KP2 (ST2 in the R1 standard) was used for dialing internal Bell System telephone numbers. However, actual frequency durations can vary depending on location, switch type, and the machine status.