How did you find out?
Have you talked about having children with her?
What are your ages?
Has she ever cheated or lied to you before?
Thank you for the additional info. Do you think its something she just hasnt had time to speak to you about? At this point you don't know what she's thinking, and there are other reasons why women are put on birth control. The best thing to do is to sit down and talk to her about it and see what she says. You have the edge of suprise in your favor, because she doesn't know that you know.
I'll say that if she has not lied or cheated on you in the past, then I wouldn't automatically jump to the conclusion that she's cheating. Of course there's always the possibility, but if she's a good woman, give her the benefit of the doubt first.
Why are the two of you no longer having sex? When did that start?
Do you want to have sex with her? Do you think she wants to have it with you?
Do you talk about it? Do you love each other? How long have you been together?
Have you ever cheated on her?
How long has it been since you had sex?
What did she say about the phone calls?