Improving Sound Quality in Phone Conversation

I've listened to it, and I don't think you'll be able to improve it at all - certainly not significantly. The problem with all of these Skype-like systems is that they inevitably distort the audio, both by limiting the bandwidth and bit depth, and inevitably introduce distortion. And without a reference to what this should have sounded like, there's absolutely nothing you can do to alter that. The only thing you might want to do is use the parametric EQ lift the LF slightly around 3-400Hz, but that really won't make a lot of difference; it will just restore a bit of the balance, and that imbalance was probably caused by the remote party's dodgy mic.

I can understand the words, and compared to some I've heard, that's quite something. Personally I'd leave it just like that, because there's a very real chance of making it sound worse by playing with it, I fear - you really don't want to go too far with the EQ.