he client the choice of how the error can be presented to the user. For example, a client with a graphical interface will likely display the reason phrase on the response, which should provide very specific information about the failure. How- ever, an audio-only UA does not have this capability (although a text-to-speech synthesizer could be used to provide this capability). Instead, an audio-only UA could fetch the URI and play the resulting audio stream to the user. If the URI is a sip or sips URI, the UA may treat the Error-Info as a Contact in a redirection response, which would result in a SIP session established to play the recording. An example is: Error-Info: k<sip:recording5@announcementsrus.com> 6.3.3 Min-Expires The Min-Expires header field is used in a 423 Interval Too Brief response (Section 5.4.20) from a registrar rejecting a REGISTER request in which one or more Contacts have an expiration time that is too short. The header field contains an integer number of seconds that represents the mini- mum expiration interval that the registrar will accept. A client receiving this header field can update the expiration intervals of the registration request accordingly and resend the REGISTER request. An example is: Min-Expires: 1200 6.3.4 Min-SE The Min-SE header field [21] is a required header field in a 422 Session Timer Interval Too Small response (Section 5.4.19). The response SIP Header Fields may also be present in an INVITE or UPDATE containing a Session- Expires header field. It contains an integer number of seconds. An example is: Min-SE: 480 6.3.5 Proxy-Authenticate The Proxy-Authenticate header field is used in a 407 Proxy Authentication Required authentication challenge by a proxy server to a UAC. It contains the nature of the challenge SO that the UAC may formu- late credentials in a Proxy-Authorization header field in a subsequent request. Examples are shown in Table 6.16. 6.3.6 Server The Server header field is used to convey information about the UAS gener- ating the response. The use and co