Tbohls32, that's an excellent question. We definitely understand how important it is to monitor usage on your account and find out where that mobile number is coming from. What is the make and model of your device? What texting app are you using on your phone? Is it possible that you may have texted a short code number, such as a company text subscription?
I had the same question and then I did a test. I sent myself a picture message from husbands phone to my email and it showed up as 999-99-9999 so it is when they send a message to an email address that this shows up. It's also shows as being sent from email@example.com hope this helps