Ask HN: Why are phone numbers considered a secure personal identifier?

It seems to me that phone numbers are a horrible identifier due to the way they can be transferred between users of a carrier. Services like Ting have made short term numbers easy to use, and I often get two-factor auth messages from previous users of a number. I have debated using Twilio to create my own number pool of international numbers and a way to check my messages via a web portal instead of relying on messaging.

Source: news.ycombinator.com