The remote Debian host is missing a security-related update.
Henning Westerholt discovered a flaw related to the Via header processing in kamailio, a very fast, dynamic and configurable SIP server. An unauthenticated attacker can take advantage of this flaw to mount a denial of service attack via a specially crafted SIP message with an invalid Via header.
Upgrade the kamailio packages. For the stable distribution (stretch), this problem has been fixed in version 4.4.4-2+deb9u3.