It makes sense to have an after-hours / weekend voicemail greeting for important teams at your business so that your clients feel confident that they will be taken care of. You may also suggest alternate resources like a help forum, knowledge base, or online chat to support customers while closed, if available.
Research done by the New York Times suggests that Millennials almost never use voice mail and many don't bother checking it.

VoIP security. As touched on above, the underlying protocol for VoIP is SIP and it wasn't built with security in mind. This means hackers are taking advantage of several new attack vectors. One example is a denial of service (DoS) attack specifically on a phone service rather than the network as a whole. This will garble or drop your service because it results in your connection quality degrading, heavy latency, and system crashes. Caller ID spoofing is also becoming more common. This amounts to a phishing attack using a phone rather than an email. By calling workers with a caller ID that looks like it's coming from inside the company, hackers are able to get employees to cough up all kinds of sensitive information because they think they can trust the person at the other end of the call.
That's the basics of UCaaS, but the concept is constantly evolving to include more communication and collaboration technologies. Those capabilities also get tweaked to provide new benefits, sometimes general, sometimes aimed at specific verticals, like healthcare, for example. The key is integration. Voice is becoming integrated with other back-end apps, and UCaaS is making that easier. In fact, it's become so popular that it's seen rapid growth over the last several years as recent research from Statista bears out.