Turn message forwarding off and nothing will happen, I've lent my cell # to broke friends afew times to make a Google Voice account, and have a Voice number myself for Craigslist exchanges, lapses between paying my carrier, etc
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.
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.
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.