Coming from Sprint the "1" key is programmed to the old owner's number because that's how you dial voicemail (you call your own mobile from your mobile). On mine the first 2 speed dials (VM and customer service) cannot be changed, but maybe on yours they can. Go to phone, menu, speed dial, and long press to see if an edit box pops up.
Otherwise a hard reset may fix that as it is probably drawn from the activation info elsewhere on your device.
Can't say if a hard reset will mess up anything else, though.