Hi JeanQ, each phone is different when it comes to managing your messages. Can you please tell me what make and model of phone you have, and I can check and see if there is a mass delete feature.
I have the same problem on my home phone. I have been deleting hundreds of text replies to texts that I supposedly sent from my landline. My first estimate was that I had over one hundred, them five hundred, then one thousand. I have given up trying to guess how many people were contacted “from my landline.”