Nicole, who was married to the 'Mission Impossible' actor for ten years, made the call after hearing that Katie was nervous about walking down the aisle with Tom.
A source told Britain's Grazia magazine: "Nicole has made sure that she has told Katie how pleased she is they're getting married.
"She's alleviated some of Katie's worries about what it's like being married to Tom and urged her to go ahead with the wedding. Nicole usually minds her own business, but she made an exception in this case."
'Cold Mountain' star Nicole got married herself this summer to country music star Keith Urban.
Former 'Dawson's Creek' actress Katie - who gave birth to her and Tom's first child, daughter Suri, in April - is said to be nervous about marrying the 'Top Gun' star as her devout Catholic parents do not approve of his Scientology beliefs.
The source added: "Nicole called Katie and they had a friendly chat, during which she said Tom had been a great, supportive husband for ten years and she shouldn't listen to any gossip about him that suggests otherwise."
George Clooney will never run for president - because he's had too much sex.
The 'Ocean's Thirteen' star - who has dated a string of stunning women including Teri Hatcher and Renée Zellweger - is convinced his racy past would ruin his chances.
He revealed: "I couldn't run for office. I've slept with far too many women.
"I have a very keen interest in political issue, but if I decided to run for office there would be a few juicy stories dragged back from the depths."
He added: "I've also done far too many drugs and been to too many wild parties."
Clooney is not the only Hollywood actor who has ruled himself out of a race for the White House.
Robin Williams, a reformed alcoholic and drug addict, also admits he colourful past automatically rules him out.
He revealed at the New Yorker Festival: "I would never run for office - because I make Bill Clinton look Amish."
However, Williams thinks Jack Nicholson should go into politics because he would be happy to be open about his past.
Impersonating 'The Departed' star's voice, he said: "Jack would say, 'Sex scandals? What do you want? I've done 'em all. Twice. And I have it on tape!'"