Windows? Try this:
http://winlame.sourceforge.net/
Edit: Do Custom Settings, High Quality, Mono Encoding, Quality 9, Standard VB mode. Your phone recorded it in mono, no need/use making it (faux) stereo. This gives you the v9 setting for variable bitrate, with a range of 45-85kbps, and a target bitrate of 65kbps. What you'll likely end up with is much lower than the specified bitrate range (probably ~30kbps), because spoken voice just doesn't warrant a higher bitrate (it's not complex enough). It should still be essentially indistinguishable to your ears, and a VASTLY smaller file size compared to the .pcm