My bf is talking to his ex .

So my bf has been talking to his ex more lately and I didn’t mind at first because they have a daughter and I think it’s good that they’re communicating because I want my daughters sister in her life and I want my bf to be a dad to both. My worry is that he’s been communicating with her to much. I hear him in his gaming room laughing with her in the phone for hours , or he will randomly go to his car for 10-15 mins .I don’t know if it goes anywhere other than just phone calls . I asked him about it and he says they just talk about they’re daughter. But Something else has to be happening I just feel it . I want to ask his ex but she already doesn’t like me because He cheated on her to be with me when she was pregnant and i feel like she still holds on to that . Should I talk to her or keep it to myself.