Audacity can do this. I found these instructions a while back in this thread.
1. Open Audacity.
2. Open the mono Wav file.
3. Click on the bottom left corner Project Rate icon to change the sample rate to 8000.
4. Go File-Preferences and click on File Formats. Under Uncompressed Export Format, choose other.
5. Choose WAV (Microsoft) for the Header and U-Law for Encoding. Hit OK.
6. Now just select the audio file and use File-“Export As WAV” Choose the location and save.
Once I did all of this, the phone messaging system accepted the file.
John
MacPro 8-core 2.8GHz, 16 GB RAM, OS 10.9.4, QT10.1, Kona 3, Dual Cinema 23, ATI Radeon HD 5870, 24″ TV-Logic Monitor, ATTO ExpressSAS R380 RAID Adapter, PDE enclosure with 8-drive 6TB RAID 5
FCP-X 10.1.3, Motion 5.0.7, Compressor 4.0.7
FCS 3 (FCP 7.0.3, Motion 4.0.3, Comp 3.5.3, DVDSP 4.2.2, Color 1.5.3)
Pro Tools HD 10 w SYNC IO & 192 Digital I/O, Yamaha DM1000, Millennia Media HV-3C, Neumann U87, Schoeps Mk41 mics, Genelec DSP Monitors, Prima CDQ120 ISDN