The reason is that, thanks to the proliferation of faxes, computer modems and mobile phones, France's 11-year-old system of eight-digit numbers is almost exhausted. The new system will have 10 digits, and - it is hoped - will provide sufficient capacity for the next 30 years.
As of 18 October, France will be divided into five regions, each of which will have its own prefix - from 01 for Paris and the surrounding Ile de France, to 05 for the south west - to be added to the existing numbers.
The change will remove one of the oddities of the earlier system, which required the prefix 16 when dialling between Paris and the provinces.
France Telecom is using the opportunity to make two further innovations, both required by European Union regulations. In the first, also due in October, France will become one of the last EU countries to adopt 00 as the prefix for dialling abroad. And next February the emergency number changes to 112.
Explaining the change yesterday, the Telecommunications Minister, Francois Fillon, said that the eight-digit system had been expected to last only 10 years.
The government anticipates a further demand for numbers after deregulation in 1998, when some subscribers may forsake France Telecom for competing companies. It also wants to have sufficient capacity to offer the possibility of portable numbers for individual subscribers and, from 2001, to incorporate mobile phone numbers into the system.
France Telecom is setting up a helpline to assist those who might be confused - but, of course, you need to know the number. For reference, it is 5211.