Not able to make voice calls When 3G is ON

[Android Question]

Hello

I am having Samsung Galaxy S. From fast few days when 3G interenet is ON then I am not able to make calls.When I turned off 3G then it works fine. Any idea about this issue.I have also re installed the software but still problem persists. One more thing when 3G is on then first calls I am able to make but subsequent calls not able to make. Kindly help.

//Anand