use parametriced Job. You don´t need to take care about the format.
B4X:
Dim j As HttpJob
j.Initialize("", Me)
j.Download2("http://192.168.1.239/Generic_Handler_JSON/HandlerVBPhonechange.ashx", Array As String("customerId",Oldn.Text,"customerId2", Newnumber.Text,"code", Codeolda.Text,"code2", Codenewn.Text))