I have the same problem with the Serene that you do. However, with my Serene the echo occurs for the person on the other end of the call. They can hear a delay of themselves. Like your echo, it does not happen all the time and does not happen throughout the entire call.
It is VERY annoying for the person I am speaking with and to be honest it is very embarrassing for me. I get criticized for buying a phone that costs so much and have been asked what features does it have that makes it so special.....I usually respond with "while it doesn't have any exceptional technical features...it looks and sounds great"....then the echo starts leaving me with nothing to say except "well at least it still looks good."
I have tried to think of everything I can to solve the echo problem. I got a new SIM card, switched carriers, etc. Nothing worked....However, I have noticed that the echo does not occur if I use a Bluetooth headset. Bang & Olufsen was nice enough to swap out my phone for a new one. However, the echo still occurs on the new one....but admittedly not nearly as often. If you ever find a solution.....I'd love to hear about it. I'm such a die hard Beonut that I'll still use the phone...even if it does have problems.