Dynamics GP Web Service "System.UnauthorizedAccessException" while fetching Vendors

Question

I am using Microsoft Dynamics GP 2013 Web Service to fetch All Vendors. But I always get System.UnauthorizedAccessException. I also added user to Web Service role assignment still same issue. My code looks like:

DynamicsGPClient gpClient = new DynamicsGPClient();
Context context = GetGPContext();
VendorCriteria criteria = new VendorCriteria()
{
    IsActive = new RestrictionOfNullableOfboolean() { EqualValue = true }
};
VendorSummary[] vendors = gpClient.GetVendorList(criteria, context);

Any help would be appreciated.

Thanks