The remote Debian host is missing a security-related update.
Henning Westerholt discovered a flaw related to the To header processing in kamailio, a very fast, dynamic and configurable SIP server. Missing input validation in the build_res_buf_from_sip_req function could result in denial of service and potentially the execution of arbitrary code.
Upgrade the kamailio packages. For the stable distribution (stretch), this problem has been fixed in version 4.4.4-2+deb9u2.