A few months back I wrote about Enabling Promiscuous Mode on vSphere 4. Well, I have learned some more since then. There isn't much information out there about recording a VoIP stream in a virtual machine environment. I hope to change that today.
First thing is first. If you are using VLANS on your physical network set your Virtual Machine Port Group to be a trunk. Also set the port on the physical switch to be a trunk instead of an access port.
Second thing, if you are using the E1000 NIC driver in your guest OS, turn off VLAN support. The screen shot is from Windows 2008 R2 64-bit, but it is similar in every other Windows OS.
Performance Best Practices for vSphere 4