Broadcasting watchdog Ofcom has launched an investigation into The X Factor on whether it breached product placement rules.
The ITV series ended on Sunday night, with Rak-Su crowned winners.
The regulator is assessing whether the show broke rules in a previous episode, on October 21, featuring judges' houses.
A Three mobile phone was shown in several shots which involved both Sharon Osbourne and Louis Walsh.
An Ofcom spokeswoman said: "We're investigating whether this programme broke our product placement rules."
Since early 2011, TV programmes made for UK audiences have been able to contain product placement.
But references to placed products, services and trademarks must not be unduly prominent.
Product placement is allowed in films, including dramas and documentaries, as well as TV series, including soaps, entertainment shows and sports programmes.
The phone was featured prominently when Osbourne sent her contestants a text and when fellow X Factor judge Walsh told his singers a location they would be heading to.
A contestant was also given a branded phone to call home after discovering that they had got through to the next stage of the TV show.
