This is a brave new world when it comes to cell phone service. Gone are the days of massive, one-sided contracts that kept users stuck with one company and one phone for the rest of eternity. These days, customers can prepay for service and switch companies whenever they want. Calling Mart exists for these travelers of talktime and data. They've got discounted refills for all the biggest carriers in the nation, including T-Mobile, Verizon, AT&T GoPhone and Net10, plus calling cards for reaching out to friends and family in other countries.