It's more about sinking that second factor somewhere that can't be redirected and only you can theoretically access by logging into Google Voice. You don't have to forward the messages to your actual phone.
but this can be configured to not forward the text/voicemail.
I hope this myth goes away soon. It is trivial for any bad actor to force-port a number, even if it is reclaimed a day or two later. If I wanted to get fired, I could yank your GV number (which is, most of the time, really at Bandwidth.com) in a few minutes; the medium-sized voice provider I am at does several of these types of ports a week to help customers trying to leave recalcitrant providers.
1. It should just be plain unnecessary to apply such a hack.
2. There is still a single weak point for all services.
3. While this prevents against the run-of-the-mill "attacker activates another SIM card for my mobile number and steals my crypto cash from $online_wallet" coming up here every few month, a more sophisticated attacker can just route the SMS to himself via SS7 (I suppose the SIM variant is easier to pull off with less traces, and obviously requires much less technical expertise).
The main issue with Twilio numbers is that they can't receive from short codes. If the service you're using sends 2FA codes from a short code, they'll just disappear into the ether.
Of course, if you're just using this as a placeholder because the site requires SMS backup, then maybe it's ok that you can't receive on them.
One thing you can do to mitigate that problem is to create a Google Voice number. Those are harder to port as long as you created it on Google Voice.