Voicemail forwarding numbers - Tmo prepaid plan
At the suggestion of a forum member elsewhere, I called TMobile and asked them about their new "uncarrier" plan.  It is a non-contract, postpaid plan with the exact same pricing and services as my current TMo prepaid plan, but I regain the ability to forward voicemails among other things.   You have to submit to a credit check for a possible deposit (max is only one month's plan ie $50).  

So going to the "uncarrier" plan was the solution for me.  Hope this helps.