Cisco 9900 Unified IP phones allow remote attackers to cause a denial of service (unregistration) via a crafted SIP header, aka Bug ID CSCul24898.
https://exchange.xforce.ibmcloud.com/vulnerabilities/90236
http://www.securitytracker.com/id/1029596
http://www.securityfocus.com/bid/64770
http://tools.cisco.com/security/center/viewAlert.x?alertId=32402
http://tools.cisco.com/security/center/content/CiscoSecurityNotice/CVE-2014-0658