to keep my 3 number

To avoid this error @Shereen zumari, you’ll to open an incognito or private window in the web browser on your mobile handset, then type-in the following address “switching.idmobile.co.uk”. 
🤞

If you’ve signed-in to the iD community using the “Remember Me” option, when trying to access “switching.idmobile.co.uk”, it won’t work @Shereen zumari.  

Either don’t use the “Remember Me” option when you sign-in to the iD community, or log-out of the iD community BEFORE you try “switching.idmobile.co.uk”.