A high speed Internet connection is required to "carry" your calls so if you have an Internet outage (or your ISP has an outage) then your phone service will not work. Internet services have improved significantly in the last few years and outages tend to be much less common than they use to be. Again, if you have a cell phone then this may not be an issue for you.
Vonage is probably one of the most well-known PBX providers on the market—and for good reason. Vonage phone service is easily scalable, so it can keep up with your business as it grows while maintaining affordable pricing. Vonage also offers integrations with everything from Salesforce to Slack. So if your business outgrows your current customer relationship management tools, you can switch without having to worry about whether your VoIP communications service offers integration with your new CRM.
VoIP service simply enables your phone calls to be transmitted and received over your home Internet connection instead of your telephone landline. This is perhaps why some people refer to the service as being an Internet phone service or broadband phone service. This does mean though that you need a high speed Internet service to be able to use VoIP for your phone service.