This is the response I got.
Code:
SIP/2.0 407 Proxy Authentication Required
Via: SIP/2.0/UDP 212.109.160.59:5060;branch=z9hG4bK31c28521;rport=5060
From: "1001" <sip:1001@sip.omnianet.it>;tag=as66afa6f1
To: <sip:190475@voxalot.com>;tag=as3ca8a686
Call-ID: 0cefea390cf655587eb649534aa63d66@sip.omnianet.it
CSeq: 103 INVITE
Proxy-Authenticate: Digest realm="voxalot.com", nonce="46aa07b5c2f912c19133ae6f638842b98e7712cb"
Server: OpenSer (1.1.0-notls (i386/linux))
Content-Length: 0
Warning: 392 64.34.173.199:5060 "Noisy feedback tells: pid=15434 req_src_ip=212.109.160.59 req_src_port=5060 in_uri=sip:190475@64.34.163.35:5061 out_uri=sip:190475@64.34.163.35:5061 via_cnt==1"