VoIP? Ages ago (like 13-14 years ago) I used to use Vonage. It was OK, but I got into two billing arguments with them on two separate runs with them, and never returned. I don't even know if they are around anymore. Others have come and gone, and the cable companies these days who offer phone and Internet are essentially running VoIP for their phone service. Skype, too, can act as a VoIP provider, as you can get a telephone number through them.
I have used a Google Voice number for many years, getting a free number back when it first came out. I have used that as my main number, primarily through my phone, using the Hangouts Dialer for now. It is essentially VoIP. There is now an adapter you can buy, which connects to your Internet connection and lets you use Google Voice over standard telephone wiring in your home or office, so you can use telephones or a fax machine. Aside from the $45-$60 for a 1- or 2-port adapter, Voice is free to use in the US and Canada. Overseas calls require a per minute fee. Call quality has improved over the years--I rarely make calls but the few I've made (even while away from home) have worked well.