I'm trying to set up Google Voicemail for my parents, who are currently on CenturyLink and its voicemail system. I added the landline phone to their account and verified it, but there's no "activate voicemail for this number" link like there is for my T-Mobile cell. Is Google Voicemail possible for landline numbers?
You can use Google voicemail as an answering service for a landline. To do this, the phone must have "conditional call forwarding" as one of the features. If your plan only has "immediate call forwarding", you won't be able to use Google voicemail.
Immediate call forwarding re-programs your carriers servers to re-direct all calls to your phone number somewhere else. When this feature is active, no calls can ring your phone; they're all being redirected. You can still make outgoing calls, but you can't receive any.
Conditional call forwarding only redirects calls under certain conditions. Usually the conditions are when the call is not answered or when the line is busy. With conditional call forwarding, you can still answer incoming calls.
Don't try to use the Google Phones tab to set up Google voicemail for a landline. Simply use your carrier-provided codes to program conditional call forwarding to your Google number.
Yes, you can use Google voicemail as an answering service for a landline, but not with CenturyLink. CenturyLink only offers Unconditional Call Forwarding where your landline rings once and forwards.
Years ago, Sprint/United Telephone, which became Embarq and then eventually CenturyLink, offered the Conditional Call Forwarding feature that is needed for GoogleVoice. However, somewhere along the way, Embarq/CenturyLink "upgraded" their service and removed this feature.