I’m confused. Why are you the one fielding these calls? Does your mother never answer the phone? Or your Dad? Have you ever asked your Dad about the calls? It seems to me that if he was having an affair, and he knew she was calling, he’d tell her to knock it off.
I also would like to know what your goal is. Do you want your mother to know your suspicions? Do you want to find out if your suspicions are true? Do you want to protect your mother? Are you trying to keep your parents together? Are you trying to make them happy? Are you trying to avoid disruption in your family and your life? How would you describe your goal here?
If you are trying to somehow fix your parent’s marriage—well, that’s too big a burden to put on yourself. If you try, you will inevitably make things worse. The only thing you can do is to be honest. That means telling your parents what is happening, but not telling them your suspicions. It is their job to figure out what is going on. Yours is just to report what is going on.
Under no circumstances should you tell your mom you think your dad is having an affair, nor should you tell your dad the same thing. Stick to the facts. Nothing but the facts. No conclusions or suspicions are appropriate unless someone asks you for your opinion.
This helps you protect yourself, too. If there is some innocent explanation for this, you don’t come across as a fool or a meddler. Stick to the facts. Let others interpret. And do not be trying to be a sleuth. You’ll just drive yourself mad. You are not a sleuth. You are not a lawyer. You are not a judge. You are a witness. That’s more than enough.