The remote host is missing an update to asterisk announced via advisory DSA 2276-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%202276-1

Paul Belanger reported a vulnerability in Asterisk identified as AST-2011-008 (CVE-2011-2529) through which an unauthenticated attacker may crash an Asterisk server remotely. A package containing a null char causes the SIP header parser to alter unrelated memory structures. Jared Mauch reported a vulnerability in Asterisk identified as AST-2011-009 through which an unauthenticated attacker may crash an Asterisk server remotely. If a user sends a package with a Contact header with a missing left angle bracket (<) the server will crash. A possible workaround is to disable chan_sip. The vulnerability identified as AST-2011-010 (CVE-2011-2535) reported about an input validation error in the IAX2 channel driver. An unauthenticated attacker may crash an Asterisk server remotely by sending a crafted option control frame. For the oldstable distribution (lenny), this problem has been fixed in version 1.4.21.2~dfsg-3+lenny3. For the stable distribution (squeeze), this problem has been fixed in version 1.6.2.9-2+squeeze3. For the testing distribution (wheezy), this problem has been fixed in version 1:1.8.4.3-1. For the unstable distribution (sid), this problem has been fixed in version 1:1.8.4.3-1. We recommend that you upgrade your asterisk packages.