The voice-activated recorder is the number one way to get evidence of the cheating.
If all they do is text, it won't work.
Since you don't have a clue, you really don't have a basis to say that.
There are a lot of apps that will hide call and text usage - they will show as data usage, not as a call or text.
What you describe sounds like she deletes some texts and is using a cheater app to hide some.
I don't see how the voice-activated recorder can hurt. It might not help, but it can't hurt. Buy a couple, put one in her car, swap it out with the other one every day, and give it a week or two.
Sounds like she has close girlfriends at the school. She probably is confiding in them about the affair, or they are encouraging the affair.
The VAR in the car will give you the truth. You will either hear her in the car with her affair partner or one of her girlfriends, or you will hear her talking on the phone to her affair partner or one of her girlfriends.
If you are going to focus on any given phone number from her phone bill, it is typical for the cheaters to contact the affair partner first thing in the morning and then also be the last one they contact before bed every single day. Send a message like, "good morning sweetie, I love you, see you soon at school," in the morning, then "I love you, nite-nite, sleep tight, sweet dreams, I will be dreaming of you" before they go to bed at night.
Do not discount the chance that she could be involved with a woman. If not an affair, a "deep" friendship with a woman who is lesbian or bi.