Hi,
Same problem here (robot/synthesizer voice) and this is my story and also what resolved the problem for me:
I'm from Belgium and I'm using Mobistar as provider.I previously owned Iphone 1,3 and 4 and never experienced the slightest problem.
On the 10th of april (2 weeks ago) I went to the mobistar shop and bought myself a Iphone 5 (since Iphone 4 works with a "Micro" sim the guy of the shop gave me a new "Nano" sim also).
After the 2nd call I made I had exactly same problem as most of you: robot/synthesizer voice, pieces of conversation missing making it impossible to understand my call partner (he on the other hand understood me).
On rare occasions 2 out of 10 calls, the conversation was normal, but 8 calls out of 10 I had to hang up.
No matter if LTE was on/off, if WIFI was on/OFF, if case was around phone or not, tried almost everything possible.
Some people said it mostly happened for incomming calls and not for outgoing calls --> for me it happended for incoming AND outgoing calls.
Anyway, 2 days later I was back in the store explaining my problem. The guy didn't have a clue about what was going on and decided to send the phone back to Apple, he said I would have to wait 1 weeks before getting some news.
Meanwhile, I used the NANO sim card with a sim card adaptor in one of my old Nokia phones and everything worked fine --> SO FOR ME IT WASN'T A PROBLEM RELATED TO THE SIM CARD OR PROVIDER --> IMPORTANT FOR THE REST OF MY STORY.
After 1 weeks I received a call from the shop telling me I can pick-up a NEW Iphone 5.
Once home, I switch the Nano sim back from my old Nokia to my new Iphone 5 and after 1st call --> same problem (impossible to understand 1 word of what my calling partner said).
Very annoyed, I went back to the store and asked to see the manager and explained my problem to him and told him I want my money back.
He agreed, but only after he tested the device one afternoon WITH HIS SIM CARD.
Few hours later he called me and told me he made and received around 7 calls that afternoon and that he had absolutely no problem whatsoever.
So I went back to the store and tought by myself: 'LAST THING I WILL TRY IS A NEW NANO SIM CARD'.
I retrieved the Iphone 5 and asked the manager to give me a new Nano sim, hoping this would do the trick.
It has been 2 days now, made and received around 30 calls and I didn't had the slightest problem.
SO FOR ME IT WAS A NANO SIM CARD RELATED PROBLEM AND AFTER GETTING A NEW SIM, THE PROBLEM WAS RESOLVED !!!!
HOPE THIS WILL HELP OTHERS, BECAUSE I KNOW HOW FRUSTRATING IT IS TO HAVE A BRAND NEW PHONE AND NOT BE ABLE TO MAKE A SIMPLE PHONE CALL WITH IT.
Regards to all