So my company decided to migrate office suite and email etc to Microsoft365. Whatever. But for 2FA login they decided to disable the option to choose "any authenticator" and force Microsoft Authenticator on the (private) phones of both employees and volunteers. Is there any valid reason why they would do this, like it's demonstrably safer? Or is this a battle I can pick to shield myself a little from MS?
Your employer might use MS Authenticator but still let you do call or SMS 2FA. If you use a VOIP number, it won't be vulnerable to SIM card swapping attacks.
SMS auth is going away, it is not considered secure in the last few environments I have worked in
OP is looking for an alternative to MS Authenticator. If this works as an alternative temporarily, they may still consider it worth it.
Yes, SMS 2FA is usually not secure due being vulnerable to SIM card swapping attacks, that's why I explicitly recommended using a VOIP number, which would not be vulnerable to SIM card swapping attacks.