Representatives of the UK's small business community have warned of impending chaos when millions of mobile phones numbers change at the end of this month. Up to four million mobile phone or pager numbers that do not begin with 07 will no longer work from Saturday 28 April. Currently, these older mobile numbers -- beginning 03, 04, 05 and 09 -- work alongside 07 numbers, but from the 28 April they will no longer function. The Federation of Small Business (FSB) is concerned that many people are unaware of the change. "This will cause a lot of confusion and result in financial losses to many businesses," warned an FSB spokesman. Users who lose their existing number will be given a 07 number instead. The changes, part of the Big Number operation, are intended to make more numbers available in response to the boom in mobile phone use. Big Number is an attempt to better organise the UK's telephone numbers by grouping all mobile numbers under the 07 prefix, all special rate numbers under 08, and all premium rate numbers under 09. To find out what your new mobile phone number will be, and those of your friends, colleagues or business colleagues, visit www.numberchange.org, or call 0800 224 2200. Take me to ZDNet's Small Business Special. Have your say instantly, and see what others have said. Click on the TalkBack button and go to the ZDNet News forum. Let the editors know what you think in the Mailroom. And read what others have said.