Phone Power - Recommend
It's worked well for me. I liked that I didn't have to buy the adapter and they will always replace it if it ever malfunctions. Do make sure not to cancel your old number before you have it working with Phone Power if you aren't getting a new one. I almost did that and that wouldn't have been good.
I did my shopping around before I went with Phone Power and in my opinion, it's hard to beat the price considering all the features included like the second line, mobile apps, and softphone. I can't see how anyone will have landlines before too long now that we are able to use our home service on any device through VOIP. There aren't any long distance charges. I don't call internationally, but they do include some free international calling too.