I would assume you are with MetorPCS since you have the Coolpad Quattro 4G. Here are the steps to change your voicemail password:
1. Dial "611" from your handset to access your myMetro phone-based system, which displays on your phone's screen.
2. Press "Option 3" for "myMetro Settings," then press "Option 3" once more to access the "Reset Voice Mail PIN" option.
3. Enter your current voice mail PIN when prompted for verification to proceed.
4. Confirm you want to change your voice mail PIN by selecting the "Yes, Reset PIN" option. The system generates a new temporary PIN automatically. Write down the number and end the connection to myMetro.
5. Press and hold the "1" key to access your voice mail system. Enter the temporary password when prompted, then choose a new voice mail password to use from this point forward. Confirm the new password, and continue to access your voice mail system or end the call. source
But in case you're with a different carrier, let me know so I can provide the correct instructions.