Default Weird problem. please help

Hello friends
There is one weird problem that i am facing.
I am on Vodafone India.
Now when i am browsing web using Edge connection and when some one calls at the same time, that person get the message that the user is not reachable.
Has any one faced this problem?
Is this only vodafone specific?
Please help.
Thanks all.
HTC Desire Z....
Another masterpiece from HTC...
HTC HD2 sim free, a work of craftmanship