Significant delay in voice calling - both on wifi and without.

I also have this issue intermittently and normally only when I call fixed lines. It's very random even in the same areas with very good 4G coverage. It's really annoying and I am suprised there aren't more complaints about this as even seems to occur with latest devices. Maybe not many people call fixed lines, but I am having this issue on/off for the last year. This issue does not happen on 3G but with the switch off just around the corner - then my experience is only going to get worse. I thought over time this would be fixed but since it's been a year already, it's not looking likely.