Haberman told Gigaom that "All three outages were caused by problems in Verizon’s service delivery core — in telecom-speak called the IP Multimedia Subsystem (IMS) .. The first outage on Dec. 7 was caused by the failure of a back-up communications database. The second, last week, was the result of an IMS element not responding properly, while Wednesday’s outage was caused by two IMS elements not communicating properly .. once each problem was fixed, it never recurred. Every subsequent outage is a result of a new bug, and it just so happens that December was the month many of these bugs chose to reveal themselves .. "
"Veizon’s IMS systems are a complex network of databases, servers, routers, gateways and policy managers supplied by multiple vendors. Alcatel-Lucent, Nokia Siemens Networks, Acme Packet and Tekelec all provide different parts, but Haberman declined to identify which particular elements or which particular vendors were responsible for the problems. In fact, Haberman defended Verizon’s vendors saying that they were experiencing the same LTE growing pains as Verizon".
For background - see "How does/will Verizon Wireless Use Tekelec's Diameter Router?" - here.