From Verizon Press Release: To help keep our customers connected with loved ones in and around affected areas, Verizon is making all calls to Japan free for most residential and wireless customers through April 10.
Additionally, Verizon Prepaid Phone Card charges for all long-distance calls placed to Japan from the U.S. will also be waived through the same period.
Check your carrier, I assume other phone companies will follow in their footsteps!