The tones the DTMF tones

when i was younger and living with my older sister – she put a physical phone lock onto the phone dial so it couldnt be used , such as the one below

This meant that the phone was unusable to make calls – you could receive calls fine . Since i was quite smitten with a girl that i wanted to call i observed that when the lock was off and i could make a call, when you dialed a 3 you would hear like 3 connections as the analogue swicthing units connected for a 3

This was exactly the same sound as you would hear when you lifted the reciever and the circuit to the telco was esatblished , if you lifted the reciver and pushed down the swicth that let the phone know you was off hook you can hear the dialling – so if you wanted to dial a sequence of numbers you could just tap them out on the phone and then you would get connected

later on in my career when i worked for a telco the analog guys used to have these swicthing units on thier desks ( like the ones below)

they where platinum tipped and so quite valuble, it was the noise of these connecting that i heard all those years ago ,when you depressed the off hook button on the phone the voltage changes went across the local loop to the exchange , where the stowgers would register the voltage change as a signalling attempt

fast forward to my latest issue – i have an audiocodes M800 gateway with a SIP trunk connection to a carrier , if i dial through the trunk to the M800 and then forward the extension to the CC where we have to enter some DTMF – the CC could not hear the tones