you seem to miss the point.

1) get US phone

2) register google voice to US phone

3) stop paying for US phone

4) you can't de-register you US phone, unless you register a new one

5a) if you never sign up for a US phone, anybody can get assigned that number and click "forgot password" on your google voice.

5b) you get a new US phone: go to 2.

that is true for every single service that ties you up to a phone number or that has phone number as recovery option.

Pay $20 and you get the number permanently. I've set up my kids' soccer club and my son's Cub Scout Pack with voicemail-only numbers. Callers leave a message and a recording and a transcription is forwarded to the appropriate person via email.

I've also read using Google Voice is highly recommended for things that require a voice or text number since it's very difficult to get hold of someone at Google that can be socially engineered. Much easier to scream at somebody at Verizon, etc.

Where can I get a permanent (US) number anywhere?

The cheapest I've seen slightly below $2/month, but all the super cheap carriers are very volatile businesses and you can't expect to keep them more than a few years.

Seeing as you don't really own phone numbers in the sense you own domain names, any permanence is limited by the life span of your service provider.

Google Voice will let you pay $20 to "buy" a phone number (either a GV number they assign, or another number you port in), which is then yours forever (subject to Google's arcane EULA, I'm sure).

...it's yours forever until it becomes Google's forever ;)

I currently have my google voice number only and not tied to a landline or cell number. When was the last time you tried #4, because it's been like this for me for a year or more.