Contacts is an application that comes with Endless. It allows one to ad contacts one at a time, or to sync with contacts from a cloud service, but had no way to import multiple contacts from a file.
Contacts however uses the same back end database as Endless OS’s built in email client E-Mail (Evolution) and it allows for import of .vcf (vcard) files. So one can populate Contacts with all your contacts by using the import data function in E-Mail.
Most Addressbook applications and cloud service contacts allow one to export “vcards” to a .vcf file. For instance google contacts has an export to .vcf option in its settings menu, you can use that to export your contacts, then import the resulting file into E-mail. And once imported into E-Mail all the imported contacts will show up in Contacts.
This can be useful if you do not wish to or are unable to make use of cloud based accounts.