Cisco IP Phone 7940 with firmware P0S3-08-7-00 allows remote attackers to cause a denial of service ("486 Busy" responses or device reboot) via a sequence of SIP INVITE transactions in which the Request-URI lacks a user name, a different vulnerability than CVE-2007-4459.
https://www.exploit-db.com/exploits/4692
https://exchange.xforce.ibmcloud.com/vulnerabilities/38853
http://www.securitytracker.com/id?1019059
http://seclists.org/fulldisclosure/2007/Dec/0196.html
http://lists.grok.org.uk/pipermail/full-disclosure/2007-December/058932.html