My parents have been using a Vonage box I set up for them probably 10 years ago at this point. It's been very reliable and they don't even have to think about the technology or hardware involved.
When I originally set it up, I disconnected the phone company's line from the outside to avoid interference and I plugged a simple phone cord from the Vonage box into an unused phone jack in their house. This makes all other phone jacks in the house perform exactly as they did with their old AT&T line. Same phones, same number, same answering machine, no difference except price as far as they are concerned.
I'm sure there are cheaper options but I hesitate to change anything since I no longer live there and they are very technically challenged. I moved out about a year after I set the Vonage box up for them and they never have to make "tech calls" to me to get things working again. I have thought about switching them to Ooma over the last couple years to cut the bill even more but I don't want to mess with a good thing.
In my house it's all cell phone. No traditional landline or VoIP necessary.