Piggybacked means that he was using someone elses phone hours for his own calls. It probably comes from the action of carrying someone on your back - piggybacking someone. They do all the work and you get where you are going.
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NewguestHi So it says that he didn't have his own phone (probably a cellphone). And if he wanted to call he used someone else's phone and number. But how then the guy who called him would know what his number is? Hmm, I'm still not sure I get it.