e case of request of authentication by the proxy server. Noting that, the UAC sends a message ACK immediately after the message of error is received. This message closes the first transaction; then the second message INVITE opens a new transaction. 21.4.3 Application of Secure VoIP The VoIP service allows users connected to an Internet supplier (ISP) to realize calls in PSTN. This scenario has the advantage that the ISP already has a security report from its client. It is 'standard', that the ISP offers this service to the client in addition to Internet access. An SIP proxy server in the system ISP will be configured as a server band of proxy for client SIP in the system ISP. This proxy server dispatch of calls to the proxy server of the Internet telephony service provider (ITSP), will select and contact the gateways appropriate to SIP. In this scenario, a possible realization of a security mechanism for the calls is as follows: The proxy server ISP employs the proxy authentication procedure of SIP to authenticate the user to call. The ISP authenticates one of its clients. Once the user is authenticated by the proxy server, the proxy verifies if the user is authorized to make a call. If it is the case, the proxy contacts the proxy server ITSP by sending INVITE. VoIP is essentially an IP system and therefore suffers the same threats inherent with all IP systems [13]. Research and development has been employed to solve the problem of the security protocols for specific applications on the IP system (the layer SSL © 2009 by Taylor & Francis Group, LLC Secure Intelligent SIP Services INVITE sip:xy@domain SIP/2.0 Via: SIP/2.0/UDP To: XY ksip:xy@domain> From: yx <sip:yx@domain> Call-ID: CSeq: 1 INVITE Contact: <sip:yx@domain> Content-Type: multipart/signed;boundary=... calg=shal;protocol=application/pkcs7-signature Content-Length: Content-Type: application/pkcs7-mime; smime-type=envelopeddata; name=smime.p7m Content-Disposition: attachment;handling=required;filename=smime.p7m Content-Transfer-Encoding: binary <envelope