Dead silence

i have an iPhone and my friend has an iPhone too but for some reason sometimes (not always) when we call each other the connection doesn’t pick up. Neither of us can hear each other and it’s dead silent on both ends. We FaceTime and can hear just fine. The auto collects don’t connect thought. Also, the weird thing is that we can all someone else and both of our phones work calling someone else. It’s only our connection that goes dead silent. These will last for a period of time as well. Then, will be fine again. Then like a week later it’ll happen again. Anyone have any ideas on the issue?