Texting function

Hi,
We have set up the text function to send a text to clients after they have been to us asking for feedback. Currently we appear to be texting a number which we cannot find on our system. The individual says they have never been to us and, understandably, would like us to stop! Any ideas on how we can track and/or delete this number from the text function?
Thanks
Clive Elwood
Davies Veterinary Specialists