My FaceTime won’t work for one specific person

I can FaceTime everyone else perfectly but when I get on FaceTime with this one person it’s like it’s no WiFi it’s very laggy but I know WiFi is not the issue because they can hear me fine but I can’t hear them it’s laggy words and I don’t know why and they say that’s everyone else can hear them fine when they get on the phone I don’t know why this is happening