Bring in numbers from Google, a CSV spreadsheet, or a vCard (.vcf) file.
Duplicates are skipped automatically.
From Google Contacts: open contacts.google.com → Export → choose Google CSV or vCard, then upload the downloaded file here.
From iPhone / Android: share/export a contact or all contacts as a .vcf file and upload it here.
From a spreadsheet: save it as CSV. A column named Phone/Mobile is detected automatically; a Name and Email column are picked up too.