I have a single analog telephone attached to to the back of my UPC-provided Ubee evw3226 modem.
When my phone is being used, people complain that their calls to this phone appear to connect (connect seconds indicated on their phone) instead of immediately indicating that the line is busy.
They have to hang up to interrupt the connection.
Many thanks for your help. I entered #555# and #888# (waiting for confirmations), and while I have not had the time to test it, I am sure this is the answer.
It is nonetheless curious that no default annoncement is given when transfering to the voicemail.