[disposeofme123]

Actually, your only half right there.

You can use it on a WiFi only device. I currently have signal installed on a brand new iPod touch 6th Gen.

Its a device for using Signal messaging and phone calls only.

On a separate smart-phone, I entered the phone number, registered, waited for the phone call and then entered the verification code in the app.

I applied as much hardening as possible to the iPod and tether it either to my regular phone via WiFi or via a dedicated 4G modem i also have.

Easy and significantly safer (i think) as it mitigates base-band attack issues.

[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.