To receive an accurate answer you need to specify which product and version is used in your contact center.
Assuming that it is Unified Contact Center Express then the answers to your question are:
1. If an agent in Ready does not answer a call presented to them the system will place them in the Not Ready state - the call will not ring indefinitely on that extension. You could configure the system to place agents back into the Ready state automatically which will sort of achieve what is required but a better option would be for your customer to manage his call center agents properly - make sure someone is always in ready and answers the phone when it rings.
2. This is standard behaviour for contact center queues. Whilst the caller is on hold they will hear music from whatever MoH source is assigned to the CTI ports used to queue the call. It can be useful to configure the system to play comfort messages thanking the caller for holding and optionally telling them their place in the queue and/or expected wait time.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.