1. Install rootexplorer or equivalent, that can use root privilege, from the market.
2. Open up /data/data/com.android.phone/shared_prefs/com.android.phone_preferences.xml
Note: If the file is not there send yourself a voicemail and it should be there.
3. Add this line: [HIGH]<string name="vm_number_key_cdma">5555555555</string>[/HIGH], without the main qoutes, where 5555555555 is your 10 digit phone number, right above the vm_id_key property
4. Save and exit.
5. Delete com.android.phone_preferences.xml.bak
You may need to reboot for it to take effect.
6. (Optional)Make a backup of the file you just modified for future use.