This could be someone pretending to be from ccsusa.com which is a legitimate company. Do not provide them with any data such as you SSN. On the other hand someone may have gone to an Xfinity store with a copy of your Xfinity/Comcast bill and other data and maybe bought a phone and signed up for Xfinity Mobil.
I presume that you have Xfinity/Comcast Internet which is a requirement for Xfinity Mobile. If so,
1.) Go to Xfinity.com and Sign In to your account. (or create online account)
2.) Under Services, select Xfinity Mobile if shown, and you will see if someone has bought any phones or has any mobile lines, and these would have been charged to a credit card. If not shown, then you have nothing to worry about.
This happened to me but I was able catch it early. I was told to place a freeze on all credit reports and report anything on my credit report as fraud. Xfinitys fraud dept took care of the problem with my account. I went to the police but because I wasn't out any actual money, they told me everything I had done was correct and all I could do at that point. The fed ex guy actually showed up with a new phone...they placed a hold on my fed ex account so they could pick it up but too late for the shipment and that's how I found out. I just got lucky. See if you can get someone here or on xfinity chat to give you the phone number for their fraud division. They are very knowledgeable.