Okay, first of all, don’t panic right away because girls can lie about crazy stuff like this, but you definitely need to get to the bottom of it. You can’t just ignore it because a baby is a huge deal, but you also can’t accuse him without proof. Ask to see the texts or the test results she claims to have before you blow up his phone.
When you ask your boyfriend, watch his reaction super closely. If he gets super defensive, starts calling her crazy, or tries to turn it around on you, he’s probably guilty. If he’s genuinely shocked and willing to take a test or talk to her with you, maybe he’s telling the truth, but honestly, my trust would be totally gone at that point anyway.
If it turns out he actually did get her pregnant, you need to run for the hills because you don’t want to be tied to that kind of baby mama drama for the rest of your life. You are way too young to be dealing with someone else’s kids and lies. Just pack up your feelings and leave him to fix his own mess.