IIRC, It can be cable or DSL...I have no idea about DirectTV.
We've been using VoIP phone service for many years with good results. We even took it overseas with us when we lived in Switzerland. We've used Vonage and then another company, SpeakEasy with good results. We had better clarity with calls via Vonage. SpeakEasy was fine, but sometimes I heard an echo when I was talking or a slight delay.
One concern many people have is 911 service. You have to register for 911, and it's simple and easy. We had no problems setting it up or using it.
Vonage - A better phone service for less
There are other things to keep in mind too...if your power goes out, your phone won't work...if your ISP goes down, your phone won't work etc
There are a lot of great features, one we like in particular is that we can get an email or text message when a voicemail comes in on our phone. We travel so much, it's nice to know when a message comes in at home.
Good site with forum, it's been helpful to me in the past:
VoIP Providers - VoIP Services - Broadband Phone Company Providers
Tons of info online...just google!