My girlfriend is still talking with her ex?

So, me and my girlfriend have known each other for about 10 months but we've been in a relationship for only three, so we have established a good amount of trust. So my girlfriend has an ex. He's a pretty good looking guy, a nice dude in general, but the thing is she still keeps talking with him. She's aware that I don't like it when she talks to him, all that stuff, but when I confront her about it, she usually turns the argument to me, saying something like: "Oh, well do you not trust me? Why do you have a problem with me talking with him?". There was a period when she didn't talk to him, but lately she started speaking with him again.
Also, recently she's been pulling these "pranks", saying "I'm in a relationship with him." and then they both tell me "Haha, we got you, it was a prank." (I'm friends with the ex so he let me in on the prank before she pulled it). Then, recently he had some troubles and he called my girlfriend for help, so yesterday I'm on a skype call with her, he calls her and she hangs up. She calls me 5 minutes later, telling me that she told him "It's okay, I love You" **** like that, and then she tells me she said it just to calm him down. A few weeks ago, she asked me: "Will you ever let me and him go out somewhere alone?"
Of course, I really do trust her, but I don't know what to do. Like I'm okay with her talking to him, but I just don't like all these pranks she's pulling on me, and also all of the stuff related with everything I talked about.