I'm not an attorney in Texas and I believe their law will control your case. You should consider seeking advice from a Texas attorney. You can find one here on AVVO.
The statute of limitations is 4 years in Texas. The time begins from the date of last activity (such as the last payment made). Based on your facts you may have a defense based on the statute of limitations under Texas law.
The statute of limitations here in Tennessee is not as short. It is six years.
The language in their letter is standard language required by the Fair Debt Collections Practices Act. If you send them a letter demanding they verify your obligation on the debt and the amount they must stop collection activity until they provide the information to you.
Often with old debt that has been purchased by a debt collector it can be settled for pennies on the dollar. It is probably worth the small expense to have an attorney negotiate this if the amount is substantial.
This answer does not constitute legal advice nor form an attorney client relationship. I am not your lawyer. If you have a legal issue in Tennessee you may contact me for a free consult.
In Tennessee, the Statute of Limitations on a Contract is 6 years from the date of your last payment. Every time you make a payment, the clock starts again.
I agree that this does sound like Texas law will control. Check with a Texas lawyer, pronto.
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Four years in Texas, six in Tennessee. Sounds like Texas is the controlling issue in your case, but if they sue you in a Tennessee General Sessions Court they will argue that Tennessee law controls. They always do.
The "mini-miranda" notice they gave you is prescribed by the Fair Debt Collection Pratices Act. Dispute the validity of the alleged debt and see if they respond. They cannot lawfully take further collection activity - including suing you - without first providing validation.
I am not your attorney nor is any answer I may provide legal advice. You may contact me directly for legal advice only if you are a resident of Tennessee insofar as I only practice in Tennessee.