Practice makes perfect. Again, another obvious one, but it has to be stressed. If you want your voicemail greetings to come off as genuine, you have to practice. If you simply throw together a greeting that sounds and feels rushed, your customers aren’t going to let you get away with it. You will come off as if you don’t really care, and that won’t convince anyone you’re prioritizing the customer experience.
There are still other legacy protocols around, and some non-SIP standards, including H.232. Other protocols that are still around are the Media Gateway Control Protocol (MGCP) and the Skinny Client Control Protocol (SCCP). The former is known for being overly complex and also that it doesn't support some of the functions that people need in their phones, like caller IDs. Cisco is the primary proponent of SCCP since they developed it. But even so, Cisco is currently phasing SCCP out in favor of the much broader SIP standard.

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.
The pandemic has hit most VoIP installations very hard. That's because what we've discussed above has been mainly about optimizing one network, namely your primary office network, for VoIP traffic. Those steps mean significant work and time, both for your VoIP vendor's engineers as well as your IT staff.