I've been changed to VoIP, and the change most definitely isn't transparent.
Previously you just plugged a 'phone into the wall socket, and it worked.
Now, you have to buy a router, you have to _configure_ it, for goodness sake, and you have to plug the telephone into that. You also have to pay for electricity to run the router, 24 hours/day, 365.25 days/year. I doubt the telecoms firms give a fig about the excess CO2 they will cause to be generated.
To say nothing of your 'phone not working when the electricity in your house fails. Great for emergency calls.