In practice, most businesses are simply handling this on a case-by-case basis since COVID is a temporary problem. Most home networks can handle the extra load as long as the employee makes sure that other latency-sensitive traffic, like gaming or video streaming, is kept to a minimum during the hours when they need to talk. If some home routers develop problems, IT staff simply build a queue and handle those one at a time. Sometimes they'll be able to access the router remotely with the employee's permission, sometimes they'll have to walk that employee through configuration steps to fix the problem. Sometimes the employee will just have to live with it unless the company springs for a new router or a higher bandwidth tier from the employee's ISP.
This is a tough category because all telephone service companies have complaints on public forums. Remember that people who comment on these forums are more likely to be unhappy with the service, while happy people don't frequently take to publicly announcing their happiness with a product. Acknowledging that all the companies we looked at have some complaints, the winner for best service is Vonage, one of the first phone companies to offer VoIP for business.

You don’t want to answer calls 24/7 (unless you’re serving clients globally and there’s an expectation of 24/7 support). If you’re unavailable during specific hours of the day, use the voicemail examples below for after-hours business calls.
11. “Hello! You’ve reached [company name] support line. We’ll be happy to help with your inquiry. In the meantime, have you checked out our [website, help forum, etc.]? It may have the answer you’re looking for. If not, leave your name, number and reason for your call. We’ll reach out to you within the day. Thanks for calling [company name].” Sometimes, a caller likes to find the answer to their own questions. Let them discover by directing them to your website or help forum if you have one.