There’s no legitimate service that will show you someone’s past locations just by entering their phone number—most of those “track by number” sites are scams. Legally and technically, you need the device owner’s consent (or a court order) and a tracking app installed on the phone.
On iOS your best bet is Apple’s built-in Find My and Family Sharing:
• Have the other person share their location with you (they control when it’s on).
• You can see real-time location and a basic history in the Find My app.
If it’s for a child’s phone, Apple’s Screen Time and location sharing work great together—no sketchy third-party needed. For broader parental monitoring you could install a dedicated app (mSpy, KidsGuard) with their permission.
Android alternatives exist (Google’s Timeline in Maps), but Android’s ecosystem is more fragmented and security updates aren’t as consistent—so you’re often relying on third-party apps that can be hit or miss.