I bought a Nokia 3310 3G. In the feature list it found that it supports importing contacts from a vcf file. However in practise it didn't work. To save you some time researching, here the procedure:

• Login in the nextcloud webinterface and go to contacts
• Click on All contacts and then on settings
• Choose download. This will save a *.vcf file with all your contacts. The format of the vcf file is version 3.0/4.0 (depending on your version), however the nokia 3310 supports only version 2.1. Thus a conversion is needed. I wrote a small utility to do this.
• Use the utility to convert the downloaded file to the 2.1 format
• Connect the phone with an usb cable and choose storage
• Put the file in the 2.1 format on the external card in the folder vCard
• optional: delete all existing contacts
• Use the File application on the phone to locate the file. Against intuition, you don’t need to select open but “save vCard”. The process is fast and unspectacular (you don’t see a message at all).
• Open the contact app again and enjoy your contacts :D

Sources from research:

• Parsing/converting Nokia "Smart Feature OS" backup .ib files?
• ez-vcard
• Video & Tutorial: How To import contacts to Nokia 3310 (2017) / 215 / 220 / 225 / 230
• Import Vcard (.vcf file) to Nokia 3310 3G phone