i was doing the mission for little jacob where you have to deal with the rival drug dealer, and as i'm following him through the alley the first time, my phone goes off. i answered it as quick as i could, and it's roman with his "hey cousin ket's go bowling!" bs, and niko says "shit, roman! i can't talk now!" and hangs up on him, but the target had already spooked.
i ended up letting him get too far away, so before retrying, i put the phone on vibrate, but it never happened again.