the battery on my phone is only lasting 3-4 day in standby mode
Call your phone. Once you hear your voicemail greeting, press the star key (*). You will then be required to enter your voicemail passcode. If passcode has been entered correctly, you will be able to access your voicemail menu. Instructions vary depending on the carrier you're with. If you have a bit of difficulty, you can always contact your carrier's customer service for assistance.