you would have to do some research, but did Worldcom sell their wireless unit to AT&T? IF so your contract is simply transferred and still in effect.
It was the same when AT&T wireless was sold to Cingular, the AT&T contract are still in effect and enforceable.
You would need to ask your self:
Was your service discontinued with out you requesting so ?
Did the Bankruptcy Court (or other court of competent jurisdiction) order those contracts null and void?
Or did you just figure they were bankrupt, and so you went out and got a new phone without further thought on the matter. If your answer is the later, then most likely the debt is still valid.
Don't meant to take sides, but there are deeper issues in this one.
When a company you have a business relationship goes bankrupt, never assume your are released from your obligations as a result unless you are notified by either that company or a court of competent jurisdiction. Often they will sell accounts , contract, etc in order to generate cash to pay their own creditors Sometimes these sales are even done by order of the Court as a means to pay off other debts.