You use that old method of the stick on recording pickup that Radio Shack sold for years and years.
http://www.google.com/search?hl=en&q=voip+telephone+recording finds solutions you can pick through as well.
I have a Vtech IP-8100-1, which is completely wireless - no telephone clip cords. My messages are on a Vonage site. Someone explained to me that these messages never get into my computer. I would like to record some of my calls. There probably is no software solution. Radio Shack has three gadgets which require telephone cord connections through either the base or the handset. Would I have to revert back to an older phone?