| If you type in *10# on your landline phone (not mobiles), you will be told the phone number of your previous received call.
This is very handy to determine who called you when you were out. It is still handy even if you have an answering machine because people don't always leave messages. It costs nothing to use this facility.
Hmmm ... this has received 4 downvotes in the space of 5 minutes ... if someone would kindly inform me what is wrong with this w/u that would be appreciated. AFAIK, it is a factual node which fits within E2s criteria | 
