I'm usually wary of such apps because they tend to harvest a LOT of personal data. Looking at TextNow's listing in the App Store ... yeah forget privacy with that app, and it also tracks you when you're not using it. HARD PASS.
You can use FaceTime to place AUDIO calls, not just video calls, to other Apple users, which is a vastly preferable alternative but not well suited if the person you want to call is on some other device.