Apple users have been paying money for a ringtone on the iTunes Store that comes free with all BlackBerry phones.

No, BlackBerry hasn't tricked people as part of a viral marketing campaign. It's related to the hit Netflix show "House of Cards," a TV series about politicians in the US. Just like in real life, most of the politicians in the show use BlackBerry phones.

Here's a still from the show depicting lead character Frank Underwood with his trusty BlackBerry:

And here's what the ringtone sounds like:

What you know as Frank Underwood's ringtone is actually the default BlackBerry "Spirit" ringtone that comes free with all BlackBerry phones.

Ringtone companies are selling knockoff versions of BlackBerry Spirit because "House of Cards" has made it so distinctive.

While the BlackBerry ringtone is still free, there are lots of versions of the "House of Cards ringtone" up for sale on the iTunes Store. For example, this is what you see when you search for "House of Cards" in the ringtone section of the iTunes Store: