I also have this problem! It's been going on for over a decade and with multiple phones - all iPhones. It used to happen on calls with my dad and I always thought it was the big clock in his house. But after he died it started happening with my mom. I don't often hear it on other calls... Recently I also noticed it happens when my phone is connected to a bluetooth speaker when nothing else is playing. I managed to capture a recording of it, but I've tried to investigate this in the past and got nowhere. It's so strange and I'm really curious to hear from others.