There are many factors that influence Voice over IP (VoIP). Generally speaking, VoIP is sensitive to packet loss...
and latency so you need to provide sufficient bandwidth for the number of VoIP connections that you will be using. Also, a good ISP helps. As for your VPN, it depends on the type of VPN technology that you are using. If you use a VPN with your VoIP solution, you must account for extra bandwidth due to packet overhead. I have seen tests that state that Secure Socket Layer (SSL) VPNs actually improve VoIP quality due to Transmission Control Protocol (TCP) encapsulation. Besides throwing bandwidth at the problem, treating VoIP traffic with a certain quality of service (QoS) is recommended and will help your VoIP quality. For an Internet Protocol Security (IPsec) VPN that means the client should be able to identify VoIP traffic and prioritize it accordingly.
For more information, view our VoIP over WAN tutorial.
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