Guys, Cheating or not?

Is it cheating for my partner to have a personal cell phone number of a woman I don't know and to be saying "hey sexy" and "send sexy pics"?

That's what I just discovered on my partner's cell phone. We have been together for 10 years and have 5 children together.

It feels terrible and like cheating but I don't want to overreact.

Guys, Cheating or not?
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