I recommend looking up the CDTFA phone number separate from anything that might be listed on the letter and giving them a call to inquire about it. They should be able to determine the validity of the letter.

CDTFA is the department that collects Sales and Use tax from resellers in CA.

If you don't know who EG Communications is, but it came to your address, then it sounds like someone gave the CDTFA the incorrect mailing address for their  business.

If you ARE EG Communications and you sell merchandise in your business, you must register with CDTFA and file sales tax reports each year.

You need to get in contact with CDTFA and find out what exactly is required from you.