This indicates an attack attempt to exploit a Denial of Service vulnerability in Digium Asterisk.
The vulnerability is due to an error when the vulnerable application handles crafted SIP messages. A remote attacker may be able to exploit this to cause a denial of service condition on the affected system.
Digium Asterisk Open Source 13.x
Digium Certified Asterisk 13.1
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.