Iphone is receiving texts from strangers?

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).In other words, it's as if he's watching two people text a conversation, but from several other numbers too. Before calling our service provider, does anyone know what's going on?

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

If you bought it used, I would restore it to factory. It could have been jailbroken and then who knows what was installed on it.

Maybe the phone was activated in somekind of group messaging conversation? It's best to do a complete reset on the phone just make sure to store the stuff you don't want to be deleted on your computer (Pictures, Music, Movies ect.) That should solve the problem. If it continues after the reset, then that is a big problem. Because it's like some random stranger has your number o. O Still talk to your service provider to see what they say.

Have a good afternoon & God Bless.