Hello, I’m Nathan, and I’ll be assisting you today. My livelihood depends on your positive feedback, so your satisfaction is my highest priority.
Sorry to hear you’re having trouble.
Unfortunately this particular model has no provision for adding new handsets, nor for deregsitering existing ones.
This EL52500 was one of the more basic models and was to be sold with a fixed number of handsets, preregistered from the factory.
It does not have a capability of adding more. At this point I'd suggest returning the extra handset (which may work with other models, but not this system), and perhaps consider replacing the whole phone system with a new model that would be more capable. As a 2009 set, this model is at/past its service life anyway, will need all new batteries soon, etc.
If you'd like to replace the entire system, I can recommend some options for you.
Best regards, ***** ***** for the news,