How can somebody access ur voicemail & change your greeting without access to your phone or password (sprint)?

Nope, only guys at Sprint can do that. The reason is that your SIM Card contains information regarding connecting to a network, your cellphone number, etc. Your cellphone number is unique and the things you do in your cellphone remains in the cellphone (like when you call, you can't call using another phone using the same cellphone number as your own phone).