I finally set up voicemail through *99
I first played the recommended robocaller SIT disconnect audio filehttps://en.wikipedia.org/wiki/Special_information_tone
from the computer speaker into the phone handset, then spoke our outgoing message.
Problem -- when I call our Sonic phone number now from another phone, I hear the three tones, then my voicemail message starts to play --- but the phone continues to ring, making it hard to hear the message.
Is it possible the SIT tone is interfering with the response of the system?
I'd expect the ringing to stop when the voicemail outgoing message is played.
Question also emailed to Support.
EDIT -- WORKAROUND
Cutting down to 2 rings before rolling to voicemail (from 4 rings) seems to have avoided the problem.