Don't use CF on the phone. Let the TVA handle the call, it will ring the phone and then send to mailbox if: Transfer is set for Supervised, and Call Waiting is enabled in COS and at the phone. The TVA can do this, just not an efficient use of ports.
Another reason CG's should not sell phones. They have no idea how they function, and don't know the proper questions to put to clients.
TDA 50 would be right for this client, 824 is a crippled home system not much diff in price.