Here is a video from last years DefCon on how to use a burner cell phone to generate a bunch of calls for a Telephony Denial of Service (TDoS) attack. This allows an attacker to create a virtually untraceable and highly anonymous attack. Even with a single phone, you can generate enough calls for a long enough period, to affect a small target, such as a hospital ER/ICU, small business, a small PSAP, etc.
<iframe class="aspect-video w-full h-auto mb-8" title="DEF CON 22 - Weston Hecker - Burnder Phone" width="500" height="281" src="https://www.youtube.com/embed/EDci2gb3ruQ?feature=oembed" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen=""></iframe>Link To TDoS Video
