can someone access my voice mail without my permission because i think
my voice mail has been tampered with
.

Voicemail accounts are often given out with trivial passwords, with
advice to the user of the account to change it immediately to
something which is secret and not guessable.  Many people are very lax
about this: either they don't change it, or change it to something
silly (like the extension number with which the account is
associated).  If the password on your account falls into either of
these cases, it's quite possible that your account was hacked.

Is this an account associated with a cell phone, or an office extension?