Got a mobile phone? Evening Press features writer CHRIS TITLEY looks into why your number may soon be up...

JUST as we thought they couldn't mess around with our phone numbers any more, they're ringing the changes again.

This time four million mobile phone users will have to get used to new numbers.

From Saturday, all old mobile and pager numbers which do not already start 07 will change. Any attempt to call these numbers thereafter will fail and calls will go through to a recorded announcement.

The old numbers have been working alongside the new 07 mobile and pager numbers as a safety net since September 1999. This safety net will be withdrawn on Saturday.

To publicise the change, all the phone companies got together under the banner of the Big Number.

Howard Sandom, campaign manager for the Big Number, said: "The huge explosion in mobile phones has meant that a new numbering system has had to be introduced to cope with the demand.

"So everybody should check their own mobile numbers and those in their address books now. If they don't begin with 07 then change them. They won't work after April 28."

Be warned. It's not as simple as just inserting a figure 7 to convert the old number. Some numbers change altogether.

For example, a number beginning 0370 becomes 07770; but the prefix 0585 will become 07885. To check your own number, see the contact details below.

The Big Number changes were introduced in response to the growth in telecommunications usage in the UK. They make it possible to introduce millions of new numbers over the coming years.

The new system allows codes to be grouped into a "family" of easily identifiable services: 07 for mobile phones, 08 for special rate services and 09 for premium rate numbers.

People are urged to check that they have changed any numbers stored in mobile or fixed phones or databases, as these will no longer work after Saturday.

About 12 million land line phones, databases and mobile handsets, still store the "old" mobile numbers in their address books.

Saturday's shake-up is the latest in a series of telephone number changes. These have caused confusion amongst the public, and cost small businesses thousands of pounds in changes to signs and stationery.

David Edmonds, director general of telecommunications at watchdog Oftel, said the latest change was unavoidable.

"The changes finalised on Saturday have created 300 million mobile phone numbers.

"Switching off the old codes is the final step in a totally successful two-year programme to create extra capacity to meet future needs in the UK numbering system. It has been secured effectively and with minimum disruption for phone users.

"The UK numbering system is now much clearer. Consumers now know from the dialling code what kind of number they are calling - fixed, mobile, special rate or premium rate.

"In the UK we now have nine billion phone numbers. We anticipate that overall, the numbering scheme has sufficient flexibility and capacity for the foreseeable future."

At least Saturday may silence some of the most annoying mobile phone ringing tones, albeit only temporarily.

A survey for the Big Number found that 55 per cent of people find popular ring tones annoying.

The tune "grande valse" - made famous by the comedy programme Trigger Happy TV - was voted the most annoying ring by women.

By contrast men most disliked the Latin sounding "samba".

- To find out your new mobile phone prefix, call the freephone helpline on 0808 22 4 2000.