Google Voice (GV) "gives you one number for all your phones." that means you can either get a number from them or port one of your (mobile*) numbers to use as a Google Voice number. You can then have people call that number and have it ring on your iPhone or land line, as I do. There are a lot of options, including getting an email and/or text each time a call comes in. The voicemails are transcribed somewhat poorly, but you can often tell what the message is regarding. GV will let you answer from any assigned number, bounce the call from your cell phone to the land line, screen calls, and much more.
Voicemails can be "read" or listened to on your iPhone within the GV app, or from emails, which link back to the GV website page.
A great feature from me is that I can call clients back from my iPhone using my GV (business) number, and still receive and make calls on my cell phone number on the same device.
If you need help setting up Google Voice, give me a call.
*If you need to port a land line to become your GV number, that is possible, too. It just takes jumping through a few hoops, as I did when I moved and couldn't move my business number to my new neighborhood.