Google Voice (free) plus Skype unlimited US and Canada calling ($30 for a year) seems to be working very well for me. The Google voice rings every phone I've got when I want it to and doesn't when I don't (partly, this is for students to call me, and I don't much want to help them at 2 am). Between semesters, I can disconnect the Google voice number from everything but my office phone and not have to worry about former students having my personal number (there are some I give it to, but I much prefer to make that choice myself).
Besides, the Skype allows me to make calls from home when it's raining without having to stand outside in the deluge. Not surprisingly, cell signal is limited in an earth-sheltered house! Our home Internet is bare DSL, so I do have a wired phone line for emergency calls.
So far this is working well for me. I still need a cheaper cell solution, but I haven't found anything for less than our $70 a month for two phones.
