Something

Using INVITE request as basis request - 004F4330-45AC-DB11-8757-005056C00008@<sipphoneip>
Sending to <serip> : 5060 (non-NAT)
Reliably Transmitting (NAT) to <serip>:5060:
SIP/2.0 407 Proxy Authentication Required
Via: SIP/2.0/UDP <ser-ip>;branch=z9hG4bK6394.c266ddf6.0;received=<ser-ip>
Via: SIP/2.0/UDP <sipphoneip>:5061;rport=5061;received=169.231.27.146;branch=z9hG4bK80e5db3045acdb118758005056c00008
From: <sip:3803544586@<ser-ip>>;tag=4274
To: <sip:16267057870@<ser-ip>>;tag=as01e92009
Call-ID: 004F4330-45AC-DB11-8757-005056C00008@<sipphoneip>
CSeq: 32 INVITE
User-Agent: sv1.voip.efirehost.com
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Proxy-Authenticate: Digest algorithm=MD5, realm="asterisk", nonce="51ead7c7"
Content-Length: 0