The 85-year-old woman from the Ruhr-area city was taken aback when the caller greeted her with the standard German, "Hallo Oma, wie geht es Dir? (Hi Grandma, how are you?)
No matter than she doesn't actually have a grandson - his ignorance of her roots piqued her annoyance in the opening moments of the conversation.
"First, you don't say that here in Dortmund, you say 'Na Omma'," she ticked him off, before curiosity got the better of her and she asked what he wanted.
Undeterred, the caller explained that he urgently needed €10,000, at which point his 'grannie' said she had no money, hung up and notified the police.
Each day, dozens of senior citizens fall prey to conmen seeking money or financial information by phone, or bluffing their way into homes.
Police praised the woman for her handling of the situation, but noting that the conman was possibly shrewder than appears: "Some elderly people are lonely, forgetful, hard of hearing or just too trusting" and allow themselves to be led along, a spokeswoman for the Dortmund police told The Local.
And conmen will use various ruses to draw people into conversations before trying to extract valuable information from them, including bank details.
But apart from his use of 'Hochdeutsch', the caller made another fundamental error in Germany, she added: "German politeness is such that you don't make these requests by phone but in person!"
In the event of any unusual phone calls purporting to be from relatives or friends, police advise:
- Be suspicious if anyone asks for money by phone. Just hang up.
- Check if a claimed relative is one by calling back to any pertinent family numbers you have.
- Inform the police immediately on 110 if the call seems bogus, or if you were successfully cheated out of money.