Hi, I am having exactly the same thing, I can hear people on the other end ok, but I sound just the same as you describe to them. Only had the car 2 weeks, so its booked in for the dealer to have a look.
this is a well documented problem with older models and I had the same problem with my 2013 DStyle. I took mine into Citroen and they repaired the fault, I can't remember what they actually did but I never had the problem raise its ugly head again.
I'd take it into Citroen if it's still under warranty.