As a practical matter a phone number given for MFA will be abused for other purposes, that happened at both FB and Twitter[1,2].
If the servers or companies are in America (where I was born), GDPR will be reactive to incidents and thus good opsec is needed since GDPR can't solve a damn thing if you're dead, severely injured, or horrifically traumatized.
That's why I suggest keeping two numbers -- one in a VOIP provider. That's your Signal, that's your "personal" number etc and you can filter what hits the phone in your pocket.
Then you have a sim in your phone that changes often which the above redirects to. (Pick one with a good data plan since connecting to wifi harms location privacy)
Just be wary of VPNs if using Tor, since it will erase the benefit of having multiple circuits[3].
Also on a personal note: I had several IT failures and this setup is currently broken -- I found the password to this account after recovered a passphrase last night I thought I had permanently forgotten after a violent incident -- I hope folks don't abuse the spirit of this advice by filling my phone with death threats, spam, and photos of themselves from the neck down like the last time I gave out privacy advice in the clear
(Sorry I couldn't be more help on the policy side -- those people seem to just... do whatever they want. Be careful!!)
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[1] https://www.darkreading.com/endpoint/twitter-slip-up-spills-...
[2] https://techcrunch.com/2018/09/27/yes-facebook-is-using-your...
[3] https://web.archive.org/web/20161113094500/https://matt.trau...