Value was either too large or too small for an Int32 in asp.net

i have a singup forum in whihc there is phone number field
code is
try { cz.SignUp(nametxt.Value, passtxt.Value, Convert.ToInt32(DropDownList1.SelectedValue), Convert.ToInt32(DropDownList2.SelectedValue), Convert.ToInt32(DropDownList3.SelectedValue) ,mailtxt.Value,Convert.ToInt32(numbtxt.Value)); //GridView1.DataSource=ca.viewadmin(); Lbe6.Text = ("Signup Success"); } catch { lbe5.Text = ("FAILED"); }
but when i click on submit button it show me error Value was either too large or too small for an Int32.
i also try uint,int64 but it does not work