[edent]

If you don't have a SIM - or some other way to get a number - you can't use Signal.

If you buy a temporary SIM, and that number is recycled, a new user can register it with Signal.

[disposeofme123]

I'm assuming you already have a phone number that you'd otherwise regularly use. (does anyone not??)

If that's the case, then you can register it to Signal on an iPod and not have to worry about base-band attacks on iPhone/Android devices. That's the point.