Back in the days Gmail accounts where only attainable by invite and you had to get a new cell phone number if you switched to another service provider (at least in my country).

These days you just apply for a personal Gmail account and your cell number is yours for life.

So a username, cell phone number (plus international dialing code), or email address are all viable alternatives, although I prefer the email option.

I also agree that you must manage on which sites you use your personal or work email accounts.

In the early days of the Internet, a lot of people (including myself) made the mistake of using work email addresses for non work-related sites (internet banking for example), but these days it should not be the norm anymore.