There's no CSV import on the iPhone. Contacts, iCloud and Finder all expect vCard (.vcf) files, so a spreadsheet of contacts has to be turned into a VCF first.
After that there are three ways onto the phone. Pick the one that matches how you already sync contacts.
Save the spreadsheet as CSV UTF-8 in Excel, then convert it:
The full walkthrough is in how to convert CSV to VCF.
iPhone and iCloud read 3.0 reliably. It's the safest choice unless you have a reason to pick another version.
Contacts go to your default account. Check Settings > Contacts > Default Account first so they land in iCloud and not in an old Exchange or Gmail account.
If you already keep contacts in Google, import the CSV at contacts.google.com and add your Google account on the iPhone (Settings > Contacts > Accounts > Add Account > Google) with Contacts switched on. The list syncs across without any VCF step.
Other problems such as garbled names or missing numbers are covered in the troubleshooting guide.
Convert the CSV to a single vCard 3.0 file, then import it through iCloud.com, by opening it on the phone, or through Google sync. Switching to Android instead? See importing CSV contacts to Android.
No. Convert the CSV to a VCF file first, then import that.
Yes. Open the VCF from an email or the Files app and tap Add All Contacts, or sync through a Google account.
Usually rows with no name or a broken line in the CSV. Check the CSV in Excel, fix those rows and convert again.
Only if the VCF contains them. A CSV doesn't hold photos, so contacts converted from CSV arrive without pictures.
On iCloud.com select the imported contacts and delete them; the deletion syncs to the iPhone.
