Unless the word "line" itself refers to the cable that enters the residence.
Cell phones are routinely referred to as "telephone lines," as in "How many lines do you have on your account?" or "I want to add another line."
Even the more literal, 20th-century use of the term presents potential ambiguity. Assuming for the sake of argument that "telephone line" refers to a physical cable or wire that connects my home to the telephone network...
I can have two telephone numbers that operate on the same "line." In other words, I can call up the phone company and say "I want to add another line," and they don't have to run another wire into the house.
That probably wasn't the case way back when IRC 262 was written. There certainly was a time, perhaps as recently as the 1970s or early 1980s, particularly in rural areas, where you couldn't add a second line without having the phone company come out to the house and add additional wiring. But today, the basic wiring usually supports at least two lines.
There's even a service called distinctive ring, available from some landline companies, where you can have two phone numbers but only one line. In other words, you can't use both numbers at the same time. Both numbers operate simultaneously on the same line and the same telephones. You give out your "personal number" and your "business number," and you can tell from the type of ring which number the caller dialed.
For some people, it's a perfect solution. Costs a lot less than two separate lines. Don't need two different jacks, or a multi-line telephone. Both numbers ring on all phones in the house.
But it's not for everyone. Calls to both numbers probably route into the same voice mail box if you don't answer, so you can't have a different voice mail greeting for business calls.
And don't ask me which number is displayed on Caller ID when you make an outbound call. I have no idea how that works, or whether you can control it...
My point is that the definition of telephone line is open to interpretation, based on the context, and that the definition has changed over time...
These are things that a court would have to consider when determining whether a cell phone can be the "first line" of service to a residence.
But I'm not sure it will reach the courts any time soon. Even in a basic IRS audit, they might just accept the argument. If the person works from home and is required to use a landline for work (this was the original question), and it is the only landline in the house, and they use it only for business, and they have a cell phone for personal use...
The auditor might just accept the fact that the cell phone is the primary, personal telephone, and the landline is exclusively used for business.
Even IRS auditors occasionally use common sense.

BMK
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