I see your phone call request, but I'm not near a working phone right now. Another expert MAY respond to your request at any time.
With only one handset in the current system, and just this one "additional" handset, it would be hard to determine if the Main Base unit is preventing the addition OR if the handset itself is the cause.
Since these systems are relatively inexpensive, what I would really recommend it is returning the handset for a refund (or credit), and purchasing a "system" of the same model that has the total number of handset you want with the system.
As an example of what's out there, I have a link to Amazon where I did a search for the main model number (CS6829), where you can see a few "-2" systems (come with two handsets), and get a feel for the pricing.
Click Here for Amazon Search results --> http://amzn.to/2e93fAG
NOTE: While searching for that model number, I also saw a few listed a possible LOCAL stores - such a Walmart. So if you have any of those stores in your area, you may want to make a call and see what they have in stock.
Please let me know what you find or if you have any more questions.
Thanks, ***** *****