Ringback
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Ringback is a test used by many phone companies to quickly determine whether or not a new phoneline is working correctly and the phone number is routing properly.
Typically it works by calling a "ringback number" and then hanging up. Often the ringback number plays a low frequency tone into the line to indicate that it's working, at which point the technician "flashes" the line (hangs up very briefly) to indicate they want a ringback. This makes the ringback play a higher tone to indicate it is working, and then the technician hangs up. The phone will then ring, answering only with a tone if picked up.
The tone is added so that telephone subscribers will not be able to easily use the ringback system as an "intercom," to call other extensions of their own phone line by simply initiating a ringback test and waiting for the phone to stop ringing (so as to know when the other person has answered).
New Zealand Telecom
In New Zealand, you just need to call the number, wait for the tone, then hang up.
137(Normal Tone)
136(Continuous Tone)
Telefónica de España landline system
In some areas of Spain, users connected to Telefónica de España landline system can obtain a ringback by dialling 020 and waiting for continuous tone.
Canada (Ontario) (Presumably Bell)
It used to be if you lived in Ontario, dialing the area-code 999 and then the remainder of the phone number would drop your line back to a dial-tone, but with a higher tone than normal. Then all one had to do was hang up, and quickly return to the line to hear a normal dial-tone again. Once the user hung up once more, a few seconds later the line would ring.
No longer useful due to the requirement of area-code use at all times in densely populated areas.