My Mrs gets a call from her bank (same as mine) when she tops up her mobile for £30 - every time it happens.
Bjeezuz imagine if I was sitting next to her "oh that £30 on my card, yes that's legit....I'm fucking Sweet-Lory next Monday so had to look at her PG!!!!
This is because fraudsters commonly use this method to check if they have correct card details and if there is funds available in your account.
Because mobile top up is so mainstream the best way now for banks to check it is not a fraudulent transaction is to contact the customer.
I had a situation where someone tried to do fraud on my account in this way, they had made 2 mobile top up transactions then they tried to purchase goods in America. My bank didn't call me when they did the top up but when they tried to purchase goods in america they freezed my account and contacted me.
Funny thing at the time I only had a contract phone so if my bank had called me like how they call your wife it would never have got so far.