Legal Question in Civil Litigation in Texas

The law firm is probably acting as a debt collector for the payday loan provider. That's fairly common. Assuming, of course, that it really is a law firm. You should be able to check that out with a google search.

If you defaulted on the loan then you probably owe money on the loan (and possibly late fees and accruing interest) but you cannot go to jail for a debt. We don't have debtor's prisons in the United States. It sounds like the practices of this law firm, or whoever is the debt collector, may be illegal and entitle you to receive attorney fees and civil penalties against them for those practices. I strongly recommend you call and speak with a law firm in your area that represents individuals in consumer debts. Do a search for consumer lawyer or consumer law.