Cyber security researchers discovered a vulnerability in how Mitel products handle a specially crafted single UDP packet sent to one of its devices that could allow for a massive amplification attack. With relatively little effort an attacker would be able to D-DOS a victim by leveraging this flaw in Mitel VoIP systems. With a ratio of 4,294,967:1, an attacker could easily overwhelm a target and in one documented case 53 million packets per second totaling 23Gbps in five minutes were sent to a victim.
https://www.akamai.com/blog/security/phone-home-ddos-attack-vector