While you're waing to do this, why not go to
No More Mr. Nice Guy and read up on how you SHOULD be handling your marriage. Women hate weak men. They admire strong men. Learn to be strong, or you'll lose your marriage.
fwiw, I don't recommend calling the guy at all. What Marriage Builders recommends is that she write a LETTER, that YOU read and mail FOR her, in which she says "Please never contact me again. My marriage and my family are too important for me to ever jeopardize it again by contact with you."
It's a psychological hurdle, more so than a phone call, which you cannot control.
If she's not willing to do the letter, she is still seeing him and lying to you.