You've probably received, and hopefully deleted, phishing emails purporting to be from HMRC (the real HMRC will never email you to tell you that you're due a tax refund).
The latest scam involves text messages. You get a text from "HMRC" asking you to call them on a number they give. You call, and are indeed connected to HMRC, but you've been patched through to their freephone number via the fraudsters' premium rate one. By the time you've established that HMRC actually have no interest in talking to you, you've spent 10 minutes on the phone at an exorbitant rate
HMRC will never text you. If they appear to have done so, ignore it!