Comcast Phone Service vs. My Own VOIP Phone

If I've got my technologies all messed up, I will appreciate correction:

I am a prospective cord-cutter. Ideally, I would like to get only internet service from Comcast, not phone, not TV.

I might be interested in maintaining a landline phone, i.e., in addition to my mobile phone. It seems that I can get landline service in one of two ways:

1. Through Comcast, obviously; and
2. With my own VOIP phone (and router) and Comcast's modem.

What would be the pros and cons of these two alternatives?

Thanks for your help.