[hamid_1]

On some international calls, you now hear 'local' progress tones generated by your own exchange, not the overseas one. I noticed this last weekend when I called a number in Canada from a TalkTalk UK landline. I heard the British "burr-burr" ringing tone until the call was answered.

This has been happening for a number of years now. Since the arrival of digital telephone exchanges, analogue speech and sound has been digitised (at typically 8000 samples per second) then sent as data. The destination exchange converts the data back into analogue audio on the customer's phone line.

While progress tones can be sent in the same way as speech, it's not as efficient. Instead of 8000 samples per second, the exchange only has to send a few bytes of data to indicate the status (ringing, busy, unobtainable). The distant exchange can generate the appropriate tones. This saves data, enabling more calls to be carried on the same circuit which reduces costs.

Not all foregn exchanges are compatible with this method so you may still hear foreign progress tones on international calls. The old Phone Books used to describe the tones, or you could call the operator who could demonstrate them to you. Occasionally, when making international calls, I got a recorded announcement in a foreign language - a bit of a problem if you can't understand it. I recall one that said "ce numero n'existe pas" (=number unobtainable) and another I couldn't work out what it said, just had to try again later. In the UK, the number unobtainable tone seems to have been replaced by a "number not recognised" recorded announcement from landlines or three rising tones from mobile phones.