Someone is accessing my voicemail

I've been getting weird phone calls all day. spaced out about an hour or so. i never answer calls from anyone I don't know. Anyways after the 4th call from a different number a voicemail came up. I immediately clicked on my voicemail and entered my password but it tells me someone on a different device has already used this password and to try again later. After a few more tries with the same response I finally got through. it was an automated response saying "Press 0" over and over again. I immediately changed my password for my voicemail but I'm concerned. What more should I do?