I bought a Nokia 3310 3G. The first challenge was already to import my contacts from Nextcloud. After some research in the internet I have succeeded. Here you find a list of the steps:
1. Login in the nextcloud webinterface and go to contacts
2. Click on All contacts and then on settings
3. Choose download. This will save a *.vcf file with all your contacts
The format of the vcf file is version 3.0, however the device supports only version 2.1. Thus a conversion is needed. Maybe this could be done with the ez-vcard library. However I was lazy and just took a default KDE program.
4. Use the program KAddressBook (default KDE program)
5. Choose File/Import/Import vCard and choose the downloaded file from nextcloud
6. Choose File/Export/Export vCard 2.1 and choose export to one file
7. Connect the phone with an usb cable and choose storage
8. Put the file on the external card in the folder vCard
9. optional: delete all existing contacts
10. 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).
11. Open the contact app again and enjoy your contacts 😀
Sources from research: