IPhone receives text conversations from random people?

My dad's iPhone 4 keeps receiving text conversations from several numbers. There are outgoing messages as well, as if he sent them (but he didn't because he doesn't text).In other words, it's as if he's watching two people text a conversation. Before calling our service provider, what can we do?

By the way, he bought the iPhone used. But he's had it for a few months already and this just started happening now.

Maybe the network provider screwed up and gave 2 people the same number? That's all I can think of…

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