Princeton University study shows how fraudsters can abuse wireless provider websites to identify available, recycled mobile numbers that allow password resets at a range of email providers and financial services online. The researchers located newly-recycled mobile numbers by browsing numbers made available to customers interested in signing up for a prepaid account at T-Mobile or Verizon (apparently AT&T doesnt provide a similar interface). They said they were able to identify and ignore large blocks of new, unused numbers, as these blocks tend to be made available consecutively.”]
Source: https://krebsonsecurity.com/2021/05/recycle-your-phone-sure-but-maybe-not-your-number/