Nomorobo setup requires Advanced Call Forwarding and Allow Access to Voice Features but these are unavailable

Hi. I'm trying to set up the nomorobo.com feature to block robocalls. The instructions say to add nomorobo's phone number to the Advanced Call Forwarding section of my voice settings, but after a text box briefly appears there's an error message in the Advanced Call Forwarding section -'Sorry, but something went wrong. Please try again in a few minutes.' I tried this multiple times over multiple days.

Another help page instructs me to 'Allow access to voice features' under 'Account and identity', but that option is not displayed there. I'm the only user on the account.

Would appreciate any help you can provide.