i think the problem isn't the phone number, but the special hardware/vendor lock in that is required for it. if you travel a lot or live in a country where it is just easy to cross borders as a part of life, it quickly becomes obvious that being tethered to a regional provider for your phone number is a problem.

you end up paying ridiculous roaming fees to keep your number active in the other country, or you lose any ability for people to contact you by phone. it's incredibly frustrating when voip is so close, but not the 100% solution. couple that with providers still charging ridiculous fees to call numbers in other countries and it gets even worse.