It's been a slow burn with landlines, just like with cable. There was a bit of a growth in landlines in the late 90s and early 2000s when internet usage with dial-up modems was a growing product, and before cable modems and wireless data really took off. after that, though, second-line usage rolled off appreciably as some people converted to cell-phone only, and cord-cutters (phone version) started to eat away at the cable companies' and phone companies' client base.

There are still a lot of landlines in homes and businesses to this day, though. Ma Bell's distant grandchildren still have their core products throwing off a lot of cash (at least for now), but they've definitely noticed the roll-off in their subscriber base.