How do I back up telephone numbers

Using /e/OS Settings
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1

Can I back up my numbers or contacts from phone to sim card?

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2

I don’t think this is still possible.

But you can export your contacts into a VCF file which should be a very small file that you can easily transfer to your computer, or to an SD card or perhaps even send as an email attachment to yourself.
In the CONTACTS app, choose the “hamburger” menu at the top left, then “Settings”, then “Export”.