Verizon and T-Mobile primarily provide their voice services using cell towers.
But voice services existed before cell phones!
They were delivered over copper cables between phone exchanges and people's homes. Some people and especially businesses still have these 'landlines'.
I would imagine that most customers of the companies on this list use neither landlines nor cell phones to access their voice services. Instead they likely use some sort of IP-based voice protocol like SIP or IAX.
But voice services existed before cell phones!
They were delivered over copper cables between phone exchanges and people's homes. Some people and especially businesses still have these 'landlines'.
I would imagine that most customers of the companies on this list use neither landlines nor cell phones to access their voice services. Instead they likely use some sort of IP-based voice protocol like SIP or IAX.