With all respect, Frontier is not a joint venture between AT&T and Verizon, nor will its bankruptcy result in a return of its assets to those companies.
History time. Back in the day, every place had one and only one telephone company. Prior to the AT&T break up, only about 80% of the country was served directly by a Bell company (AT&T). The rest, mostly but not exclusively rural areas, were served by independent telephone companies. Of those GTE was the largest, but there were several more, including Allied Telephone (AllTel), Central Telephone (CenTel), Continental Telephone (ConTel) and dozens of others, including Citizens Telephone and Rochester Telephone, which are the predecessors of Frontier.
Following the AT&T break-up, and with the advent of alternatives to POTS (plain old telephone service) a complex series of mergers occurred, between the seven "Baby Bells", GTE and the other telephone companies.
AT&T and Verizon between then owned all but one of the Baby Bells. Verizon also owned GTE, which had previously bought several of the other independent companies. They both (correctly IMHO) figured that there was no real future in POTS, and started selling off lots of places, mostly by the state. The buyers were companies such as Windstream, Iowa Telecom, Hawaiian Telephone, and, most aggressively, Frontier. Starting in the 1990s, it bought what ever Verizon and AT&T wanted to sell, mostly in what others would call "fly over country".
It just has not worked. The previous owners left the systems in bad shape, the workers made and make insane above-market wages, and less and less people have POTS every day. Attempts to expand into other things like internet and TV pretty much did not work.
So now Frontier becomes, by my count, the fourth buyer of old Bell or GTE lines to go bankrupt.
The future? Depends on finding someone interested in POTS. Cincinnati Bell? Windstream? Carlos Slim?