Phoning it in

If both phones are only used for business then the expenses for both are deductible on Sch C. 

That wasn't what I said. Please reread my question.

Why would you think a 100% business phone is not deductible?

Of course, the phone itself is an asset that must generally be depreciated, although it may fall under one of the special rules that allow you to take accelerated depreciation or use it as an expense under a safe harbor.  (Which then creates a recapture issue if you sell it used when trading up.)  And the monthly bill would be a business expense.

Why do you think it might be different?

The phone bill for the one that is 100% business use, is 100% deductible as a business expense. Doesn't matter if it's a landline or a cell phone.