I did our landline (CenturyLink) to Google Voice via Ting.com in August.
My son bought a new phone, so I took his old Galaxy S5 and ported our landline to it via Ting for a month ($6/device).
Porting took exactly seven days.
I let the phone number (landline) “sit” on cell phone for a good month (bluetoothed it our Panasonic cordless phones) before I then ported it to Google Voice.
My old GV # automatically forwards to new GV (landline) until January 2018.

I have an ObiTalk 200 and set up 3 of our Google Voices on it. Getting the ObiTalk set up correctly took me a few weeks (mistakenly set up #'s in “circle of trust”).
Our old landline # now calls our home phone’s (Panasonic cordless) as well as my cell phone (via Google Voice).
I have another GV I’ve always used as a “junk/spam” phone # (especially for craigslist), I have the GV set to “Do Not Disturb”. If I’m selling something, and expecting a phone call, I just turn that option off.

Another plus with having multiple GV on ObiTalk is that I can dial out using another #, like calling a used car place with my spam GV # by just dialing **3 (it’s the third GV # on my ObiTalk) before dialing the phone number.

Once the landline # was finally ported to GV, I transferred the cellphone to FreedomPop; set up our second GV # on that. There was twice that I lost internet (once my fault with modem/router, other with CenturyLink), so I used the FreedomPop phone for data to check on DSL outages, etc.