A virtual private network (VPN) is a network that uses a public telecommunication infrastructure, such as the Internet, to provide remote offices or travelling users access to a central organizational network securely. VPNs encapsulate data transfers using secure cryptographic methods and other security mechanisms to ensure that only authorized users can access the network and that the data cannot be intercepted.
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Using VLAN's is supposabely a bit more secure.
On the other hand, if you plan to implement tighter secruity in the future then the whole concept of allowing a PC to connect behind a phone is less secure, since it is much harder to protect, and makes every phone a potential hub.
Not all Cisco IP Phones come with an additional port, but I think that most of them does...