Well, I don't think there's any miracles to be had here. As you say, it's a telephone call and I fear that, since phone lines filter out anything above about 4k, there's no way of replacing it.
I had a look at your file in spectral view and you can clearly see the original material going, as expected, up to about 4 kilohertz--but, above that, there's a generalised area of noise. My suspicion--and it's only a suspicion--is that during your double conversions (to wmv then through a third party app to something else) one of them has added the extra noise as it your original was full bandwidth.
Anyhow, best I could do was use the Marquee tool in Spectral View, draw a rectangle around the upper area (you can see a clear boundary between the programme content and the random extra frequencies) then just delete everything above just over 4 khz then delete.
As for making a phone call sound full and rich, if there was a way to do it, when I was working in TV we could have saved hundreds of thousands a year by using phones instead of far more expensive circuits!
Thanks Bob. You directed me toward the best tool to use (spectral view) and where to look to delete the frequencies that were causing the problem. I did as you suggested, deleting everything above about 4k, and that doesn't give me perfect audio, but it cuts out the ringing and lets me go on from there to improve it in any other ways I can.
I can actually hear the ringing when I play back the original file, so it looks like it got added either while we were recording or while the first encoding was going on. But no matter. At least I can get it out of there and move on.