Weird breathing sound when someone calls my phone and I don't pick up?

So for the past couple of months I've noticed something very weird about my phone. This doesn't happen often but when it does it completely freaks the person who's calling me out. Basically when someone calls me, my phone will ring once or twice and then the timer will go off as if I picked up (when I haven't) and there will be a faint breathing sound on my line. It doesn't go to voicemail, it just stays like that until the person hangs up. This has happened maybe 5 times already and there's no reason to since my phone is technically brand new (it's an iPhone XR that I got in November 2018). The last time it happened was today, I was taking a nap and when I woke up I saw I had a missed call from a friend. I messaged her and apologized for not picking up and she replied telling me that someone did pick up, and that they didn't say anything. This freaked me out a lot and I finally decided to see what everyone else thinks it may be.

"Heavy breathers" used to be much more common in the days of the landline.

That is pretty odd. One idea I have as to what's going on could be that, at some point, the phone's voicemail settings could have accidentally been accessed, leading to it recording general silence/breathing for a long time without being noticed, so it plays that instead of an actual "nobody is available to take your call" message? I think it'd be worth looking into how you could set a new voicemail message, to see if that may be the issue.