can you track someone by their phone number

Only if that person opts in, or if the line sits on an account you administer. A phone number alone is an identifier, not a tracking token — the location has to come from the carrier's network or from the phone's own OS, and both require authorization from the account holder or device owner.

The legitimate options, and what each one demands:

• Apple Find My — accepted Family Sharing invitation, Share My Location enabled on their device.
• Google Maps location sharing — they accept a share from their account, for a duration they pick.
• Carrier family services (Verizon Smart Family, T-Mobile FamilyWhere, AT&T Secure Family) — the line must be on your account and the person a family member you administer.
• Google Family Link — for a child's supervised Google account, not for adults.

All of these are visible to the person being located. That visibility is not a limitation to work around; it is the feature that keeps the tool lawful and the relationship intact.

Where a monitoring product fits is a phone you own and issue: a teenager's Android handset, a family Windows PC, a Mac in the study. Ultimate Phone Spy runs on those platforms and puts location history, geofence alerts, messages and call logs in a single dashboard (plans, system requirements). It is not an iPhone tool — Apple does not allow that level of access on iOS — and it is not a number-lookup service.

Practical expectation setting: cell-tower location is accurate to a few hundred meters; GPS to 5–20 meters outdoors and much worse inside a building. If precision matters, the question is usually not "where is the phone" but "did they arrive where they said", and geofences answer that better.