[geforce]

Maybe you could use VOIP numbers for 2FA? Many VOIP providers (i.e. voip.ms) can forward SMS to e-mail, SIP client, callback URL or an other phone number. There's a cost, but I guess it's minimal, all considerations done.

Your second scenario is like a dead man's switch. It's interesting, as it could prompt you with a daily challenge that only you can answer. But I don't see how it could be implemented in a normal person's life?

[tekstar]

I guess then the risk is whether or not the VOIP provider is more secure than my phone provider? I need to think that one through.

With the second scenario I was just thinking that, if I personally didn't receive the text on a given morning, I would know that my number has been ported and I would begin to freak out and try to race the attacker.

[geforce]

I think it's more of a "security by obscurity" thing than anything, but if the number is really unknown except for you and the 2fa provider, that would probably be "good enough".