The Proxy-Require header field is used to indicate proxy-sensitive features
that must be supported by the proxy. The Proxy-Require header field contains
a list of option tags. Each option tag defines a SIP extension that MUST be
understood by the proxy to process the request. Any ProxyRequireHeader
features that are not supported by the proxy must be negatively acknowledged
by the proxy to the client if not supported. Proxy servers treat this field
identically to the RequireHeader.