It isn't clear what kind of tax you are concerned about. The IRS would collect income taxes in connection with this transaction.
The seller of the phone (if a U.S. citizen or resident or conducting business in the U.S.) has sales revenue equal to the fair market value of the bitcoins in U.S. dollars, and if revenue exceeds expenses, there is a taxable profit. The buyer of the phone (if a U.S. citizen or resident or conducting the purchase in the U.S.) has a capital gain or loss based upon the fair market value of the bitcoins when acquired reduced by the fair market value of the bitcoins when the phone is purchased with them.
If the transactions exceeded $600, the transaction would have to be reported on the 1099 form for barter transactions.