Dear Desageda,
a friend of mine also has an Iphone 5 and he's also using Mobile Vikings.
He however NEVER experienced any problems while he made phone calls.
So apparently it is not with all the Iphone 5's.
Very interesting email you copy pasted in your post!
Please allow me to translate this one to Englisch, because there seems to be a very easy solution if you may believe the people from Mobile Vikings:
"Dear,
The Iphone 5 has foremost a bigger noise repression than other mobile phones on the market.
The HD Voice Quality should improve the sound, but the ones that don't have HD Voice Quaility should not experience any averse affects.
We have received some complaints from people concerning this issue and we have found a solution.
There have been some sound tests to compare the Iphone 4S and the Iphone 5.
The Iphone 5 has a noise reduction which is acting too strong at certain moments, causing parts of the conversation to drop reguraly.
What you can do is the following:
go to 'Settings > General > Access and activate the option 'Hearing aid mode'.
Normally things should be a lot better now."
it't possible that I didn't translate the menu description exactly because I don't have an Iphone at hand, but it should be more or less clear i guess.
Really hope this works.
Now just waiting to get my Iphone 5 back...