My girlfriend was on the verge of getting scammed at the weekend.
It's a two phase scam, and a pretty simple but clever one. This is what happens:
You receive a text on your phone from a number that you don't recognise. The text will say something along the lines of "Did you receive my new number? Sarah".
Here they are relying on the probability that you will know someone else who goes by this common name (or another common name, if they choose to use it). Anyway, my girlfriend does have a friend/acquaintance called Sarah, so she replied saying that she had not received her new number.
The next text she got from Sarah explained that she had lost her phone and if my girlfriend would mind sending her the mobile numbers of anybody that they both know so she can start to rebuild her contacts list.
Still not suspicious my girlfriend sent Sarah the numbers of the two people that they both know.
About half an hour passed and my girlfriend received another text from Sarah asking for a "favour".
The text said "Can you do me a favour please? I can't get to the shops today but need some credit for my phone. Can you buy me some credit and send me the code. Will pay you back tomorrow."
This isn't the exact wording, the actual text probably sounded a bit more genuine.
There was a bit more texting between the two of them and then my girlfriend realised something wasn't right so she contacted the Sarah she knows on the number she has always had for her and found out it was not her she had been texting.
Basically what the person is doing is sending out loads of texts to random numbers claiming to be Sarah, in the hope that some of the recipients would know a Sarah and send back the contact details of other people who know a Sarah.
The fake Sarah would then be armed a list of mobile numbers of people who definitely know a Sarah and will then send them texts asking for them to top up her mobile phone with credit.
Proof of this is... my girlfriend contacted one of the people whose mobile number she had just given to the fake Sarah, and they had already been contacted asking for a "favour" too.
I guess once they have enough credit topped up to the SIM they would then sell it for cheap... i.e. selling a SIM with £100 credit for £30 or so. They've probably got loads of SIMs all on the go at once, scamming people.
Anyway, luckily my girlfriend realised something was up before it was too late, but reading the texts it was very convincing that it was genuine. It's only with hindsight you start to see little things that don't quite make sense.
Just a heads up.