For a very comprehensive explanation to OP question, read the writings here carefully:
http://www.gruups.com/tzones/.
Also, there is a thread "BB 8320" (search from earlier this month) that talks about same....there is a hack but appears TMO has closed it, can't ask/obtain for BB enabler and TMobileWeb under the same account anymore....so "looks like" you're stuck with paying the $19.99 BIS plan.