May 28 05:11:10.749: Sent:
INVITE sip:551@192.168.51.13:5060 SIP/2.0
Via: SIP/2.0/UDP  192.168.51.14:5060
From: <sip:192.168.51.14>;tag=44E3062-1F25
To: <sip:551@192.168.51.13>
Date: Thu, 28 May 2015 05:11:10 GMT

May 28 05:11:10.753: Received:
SIP/2.0 401 Unauthorized
Via: SIP/2.0/UDP 192.168.51.14:5060;received=192.168.51.14;rport=57249
From: <sip:192.168.51.14>;tag=44E3062-1F25
To: <sip:551@192.168.51.13>;tag=as72c9de2f
Call-ID: C9EC2C43-42E11E5-B91097EE-60ED9EEB@192.168.51.14
CSeq: 101 INVITE
Server: Asterisk PBX 1.8.32.2
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY, INFO, PUBLISH, MESSAGE
Supported: replaces, timer
WWW-Authenticate: Digest algorithm=MD5, realm="asterisk", nonce="03de257b"
Content-Length: 0

Астериск "друга" не узнает и требует логин-пароль.