> Also, don't let your mobile phone number expire and someone else get it.

How would you do this, then? I believe old numbers are kept for a certain time (6 months?) and then put back into the public pool. If we had a more technical solution (think IPv6 addresses, but for phone numbers) this shouldn't be an issue in theory. Of course, having to remember something resembling an IPv6 address to contact somebody would be a pain. I would say we could use a username system, but I believe we've seen the downsides of that way too many times already.

> How would you do this

You port your number to a super-cheap carrier and keep paying for their lowest subscription, even if you don't need it anymore.

I payed for 5 years $5 per month to keep a number alive, even if I wasn't living anymore in the country that issued it.

you can register expired emails addresses too