SIP/2.0 407 Proxy Authorization Required
Via: SIP/2.0/UDP here.com:5060
Record-Route: <sip:UserB@there.com;maddr=ss1.wcom.com>
From: BigGuy <sip:UserA@here.com>
To: LittleGuy <sip:UserB@there.com>;tag=2341d
Call-ID: 12345600@here.com
CSeq: 2 INVITE
Proxy-Authenticate: Digest realm="MCI SIP",
 domain="sip:ss2.mci.com", nonce="c1e22c41ae6cbe5ae983a9c8e88d359",
 opaque="", stale=FALSE, algorithm=MD5
Content-Length: 0