Also known as analog lines, POTS is the traditional method to support making and receiving phone calls over the Public Switched Telephone network. With the many benefits of data network services, there aren't many cases where POTS lines are the best way to go. Usually they are only need when a device needs a loop start. Gradually, the ILECs are phasing out POTS service and in many areas of the country, you can't even order it, anymore. For instance, Hurricane Sandy wiped out most of the copper infrastructure in New York/New Jersey and Verizon did not repair. They replaced it with fiber and wireless infrastructure.
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.

With Panasonic, it’s your choice to have your phone services hosted in the cloud or installed to traditional landline phone lines in your closet — whatever makes the best financial sense for your organization right now. Because of its exceptional DECT cordless handheld telephone capabilities, this system is also ideal for installations where mobility is essential such as retail, manufacturing, and warehouses.
Ring Central's network is "fully redundant geographically," according to the company's website. That's a fancy way of saying that if there's a service outage in your area, the system will switch your company to an unaffected data center.