A while back I faced the situation, that I needed to use
Two-Factor Auth (2FA) at work, but had not a business phone.
The use case here is Slack. But I learned about other applications offering
the same possibilities, so I thought about writing up the steps.

Basically, the service supported receiving the second factor per SMS or via
an app on a phone.

Since I don't want to use my private phone for business related topics, I
refused to use either.

After some discussions with our IT and a bit of research I found an
alternative: gauth.apps.gbraad.nl.

This page served as a demo for a Chrome extension. You can find its
source code on GitHub, so you could run the website yourself
(even offline, since no requests to third parties are made).

How does it work?

When using 2FA many companies demand that you enter your phone number in order
to receive a SMS.

However, for some you can find an option that you "don't have a phone".
In this case, you should be presented a QR code to scan.
For accessibility reasons, there is likely a text representation for that code.
Namely a number.

You pick that number and head over to the website. On the right hand you can
see a way to add another service for generating OTPs (or One-Time-Passwords).
In order to compute those, you need to enter a number - the number which is
represented via the QR code.

But I have a phone

You can use a SMS as second factor then.
Keep in mind though, that there is a
recent trend to "steal" that number.

Alternatively you could use an app like
Google Authenticator, andOTP or
FreeOTP+, but I haven't tested those.