On April 22, Verizon Wireless will institute the fee for existing customers who purchase new, discounted devices with a two-year contract. Those who pay the full price of a new phone would not be impacted. It is the last national carrier to charge such a fee. T-Mobile charges $18 for upgrades, Sprint and AT&T recently doubled their fees to $36. The move comes as carriers’ profit margins are being squeezed by hefty smartphone subsidies, particularly from Apple’s iPhone.