A great voicemail greeting can even make people look forward to you returning their call. Whether you want to install a professionally recorded voicemail greeting, or you want to record one yourself, it helps to know what you want to say before you record. Here are some helpful examples.
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.

Grasshopper makes it easy for companies to receive calls on the mobile devices they already own. Features included in Grasshopper’s plans include extensions, call forwarding, voicemail-to-text — each without the need for investments in expensive hardware.
Make sure you keep your promises too. If you want to specify a time (which you should) ensure you get back to the customer within the timeframe.