Not sure if people know about this but I didn't know till now.
O2CallAlert is a simple easy to use alternative to Voicemail 901 giving you missed call information free of charge. If your phone is engaged or switched off , O2 will sent a free text message to you tell you who called you and the time.
If you miss more than one call, Call Alert will send you details of up to four callers with the date and time of their last call to your mobile.
You can switch on O2CallAlert at any time by calling 1710 free from your mobile.
And if you want to switch if off, just call 1760 free.
A very useful service for people that has to otherwise pay to retrieve their 901 voicemails.