If by “localize a cellphone” you mean finding its physical location from only a phone number, a normal person cannot reliably obtain the phone’s live GPS location from the number alone.
Legitimate options depend on the situation:
Your own phone: use Apple Find My or Google Find Hub/Find My Device.
Someone who agrees to share their location: use WhatsApp, Google Maps, or Apple Find My location sharing.
A business/organization: use a properly authorized mobile-device-management (MDM) system or carrier service.
Emergency/law-enforcement situation: the mobile carrier can provide location information through the appropriate legal/emergency process.
A phone-number lookup can sometimes reveal the country, carrier, or general numbering region, but it normally cannot give you the phone’s current GPS coordinates.If you tell me whether this is your own phone, a phone whose owner has given you permission, or an emergency/business case, I can show you the appropriate method step by step.