Phone mutes other people while I speak

Whenever I'm on a phone call and speak, the other person's audio cuts out until I stop talking (and for half a second or so after I stop - there seems to be a slight delay). This makes it difficult to hold a conversation - lots of stops and starts, and asking people to repeat themselves.

I thought this was just an issue with my old phone (an iPhone 6), but after getting an XR it has continued, so I think it must be a setting I haven't been able to find (I checked with the XR's previous owner, and this wasn't an issue.

I think this is some kind of audio ducking feature meant to prevent feedback if the phone is on speaker, but I almost never use it on speaker (I use Apple earpods for phone calls). Everything I can find on these forums is about iMovie audio ducking, or about Facetime lowering the system volume (e.g., a Youtube video won't play at full during a Facetime call).

Sorry if I just haven't been using the right search terms - but I'd appreciate any help with this. Thank you!