In the US phone numbers are much more open than any other means of identifying someone you want to contact.
"non-free" is a bizarre thing to say in this context. No identifier is going to be free as in speech - that would defeat the purpose. If you want to have assurance that you can keep the identifier it had better not be free as in beer either - the provider of the identifier could revoke it at any time.
Some sort of government-issued communications identifier might provide the needed benefits, but actually the phone number is the best solution to the problem that we have.
If you're relying on an email address, you're in much shakier territory. (Unless you own your own domain and email hosting solution, and unless you're a multinational corporation it's likely to be less reliable than other options.)
I'm pretty sure it's quite possible and easy to create an email address, be it from gmail or another email provider.
That email address will be valid and accessible as long as you have internet access. You can travel, use Wi-Fi or cellular data, and it will work.
You can create a new email address for free.
In that context, getting a phone number is much more limited and not free (you need to pay for a voice plan, etc.). And your ability to use said number from other countries (roaming) is limited and expensive.
>No identifier is going to be free as in speech - that would defeat the purpose.
I would say that a public key would work well as a free as in speech identifier. It uniquely identifies an address-holder, and the full software necessary to send messages using that key can be obtained and held on one's own. The problem comes when expecting an identifier to be associated with a person, since there you need a trusted authority to relate between names and keys.
In the US phone numbers are much more open than any other means of identifying someone you want to contact.
"non-free" is a bizarre thing to say in this context. No identifier is going to be free as in speech - that would defeat the purpose. If you want to have assurance that you can keep the identifier it had better not be free as in beer either - the provider of the identifier could revoke it at any time.
Some sort of government-issued communications identifier might provide the needed benefits, but actually the phone number is the best solution to the problem that we have.
If you're relying on an email address, you're in much shakier territory. (Unless you own your own domain and email hosting solution, and unless you're a multinational corporation it's likely to be less reliable than other options.)