Telephone Set Function 3 - To provide a way for the telephone company to indicate that a call is coming in or ringing

In my last two legacy Public Switched Telephone Network (PSTN) posts I covered pulse or rotary dial service along with dual tone multi frequency (DTMF) dialing service. In this post let's look at what makes a telephone ring.

When the user begins dialing the phone each sequenced number is stored in the central office computerized switch at the Local Exchange Carrier (LEC) Central Office (CO) and analyzed. The first three digits determine if the call is local or long distance. If the call is local, the switch determines if it can complete the call itself or the call needs to be forwarded to another local LEC CO that handles that telephone number. If the call is long distance, the call needs to be forwarded to the customer's long distance carrier.

Once the call destination is determined a switch on the receiving end sends a repeating 90 Vrms 20 Hz ringing signal (on for 2 seconds with a 4 second pause) called a ring or alerting signal to the receiving phone causing it to ring.

Ring or Alerting Signal

Notice the ringing signal has an inaudible frequency of 20 Hz - this is why different phones can have different ring styles.

At the same time, a ring back signal that is a mix of 440Hz and 480Hz is sent back to the caller. This signal is on for 2 seconds and off for 4 seconds and indicates that the phone being dialed is ringing. When the receiver picks up the handset the telephone goes off-hook.  The switch hook on the receiving phone closes, current flows and the CO switches turn off the ringing signals.