https://www.vice.com/en_us/article/wx55xz/your-old-phone-number-can-be-used-to-hack-you-study-finds

According to FCC data, roughly 35 million phone numbers are disconnected every year. A new study has found that the majority of those numbers remain tied to the previous owner, opening the door to a variety of attacks that can put their former user at significant risk. 

The new study by researchers at the Department of Computer Science and Center for Information Technology Policy at Princeton University sampled 259 phone numbers available to new subscribers at two major wireless carriers, and found that 171 were still tied to existing accounts at popular websites, creating significant privacy issues. Because of the way many online accounts are set up, it can be possible to hack an account as long as you have access to a phone number associated with it.