Since it's a network phone, you could potentially do a solution that involves wireshark. I don't know if there a program that will automatically pull the voice files of the packet capture or not. Seems like the company that Ired posted makes a program that does that? The panasonic network phone is MGCP though, not sip (which isn't a huge deal, but the program has to bother to have code to recognize it is all)
As well of course, perhaps ask your PRI provider about it, and if they have a recording. I presume if they do though it'll be beyond expensive.