> you do have to use a standard paid US mobile number to set it up, but do not need to retain it afterward

If someone else registers Google voice with that number, you're going to have a hard time. It'll be removed from your account, and you can't use Google voice without a US phone number.

Don't know if it is still the case, but for $25 one time fee (much less than one month of average phone bill) you could port your number into Google Voice, and keep it forever, at least until Google shutters Google Voice (which they likely won't, because it's such an incredible spying device).

You can set up Google Voice, then remove the number. You can continue to use it indefinitely without another phone number attached.