ANSWER:

It seems that this money is either a gift by someone Anonymous who took money from you unlawfully or someone who did that by mistake.

If he did it by mistake, he will come looking for it and complaining to the bank or phone company and they will contact you. 

But if it was a gift or done intentionally, you should keep it for a year and then consider it as yours to use and it would be halal for you, and Allah knows best.

QUESTION: Unknown number sent me recharge card, do I have to search for them for 3 years. the number is switched off every time I call.