There's no direct way to reduce the size of an MP3 file (it's already compressed as much as possible, according to its algorithms, anyway).

However, it could be decoded and re-encoded at a lower bitrate (with a loss of quality).

Another, simpler way is to resample the source wave at a lower sampling rate.

CD Audio is 44 KHz/16-bit stereo; most material comes at that sampling rate.

Resampling to 22 KHz would yield a file twice as small, which could then be encoded at lower bitrates (64-96 kbps, even 32 kbps, which still isn't so bad for a ringtone).

Most phones' speakers are very limited anyway, the K310 here only plays up to 16 KHz sampling rate.

Many audio players can write an output file to disk while resampling it: Winamp, Foobar2000, XMPlay, etc. On the Mac side, ITunes could convert a file to .wav, which could then be edited by Audacity or somesuch.

Some free editors are Audacity (cross-platform), Goldwave (http://www.goldwave.com) - though that one's shareware, lots of free ones for Linux (obviously), there might be tryout versions of the larger commercial editors.

Out of MP3 encoders one of the best is free, LAME, and it is cross-platform as well (actually Audacity worked with it, suggesting a download of the library). LAME can also decode MP3 files to waves.