Registering a cordless phone to el52319

Have you googled to see if the handset is compatible with the base unit? 

What exactly does it mean when they say SEE MANUAL

If you know for sure the new handset is compatible with your existing base unit, there are several method within this topic with instructions on how to force a deregister on a handset, you'll need to follow all links. 

I can not find any handsets for purchase to add to this supposedly expandable system.  The Model number EL52319 is not even valid on AT&T site for some reason. Returning this set.

First, AT&T doesn't sell the analog / cordless phones with the AT&T logo.  That is done by American Advanced Telephones, under license to use the logo from AT&T.  American Advanced Telephones is a d/b/a of V-tech.  

Return them or not, no skin off AT&T's nose.

Google el52319 to find all sorts of online information, including a manual which will have actual model numbers for the handsets which you can buy from all sorts of places... just not AT&T>

I have Googled extensively for days. No accessory handsets available for sale that are compatible with Model EL52319, I promise. I have the manual, which refers me to the AT&T store website (with link) to purchase an accessory handset EL50019, that does not exist anywhere except Walmart, where it is out of stock. I just purchased this set one week ago and need one more handset. Purchased this set because it says expandable. Will just have to buy a 4 piece, but I have already set them all up & programmed for my elderly Mother. Frustrating waste of time & energy. Wouldn't even need one more, if any of the five older versions would register to the new base, but they won't, each fails, even after following all deregistration instructions over & over again.

Several issues here. The topic is 4 years old so some information may no longer be accurate. Secondly you can't register a handset to a new base without either first de-registering it with the old base than re-registering it with the new one, assuming they're compatible. Thirdly if your old base is bad than you have to force a deregister on the old handset before you can register them to the new base.  Last I also can't find the correct handset online other than ebay. Going to add the manual is valid on AT&T's manual site, you just have to do as instructed and put the letter in that's on the end of the S/N, like it says 

All the information you need is in this topic if you follow the links, other than the fact you can't find the extra handset. 

Yes, definitely think the older age of the product I was just sold on 5-8-24 has a lot to do with it! I have done all the deregistering many times, so I guess previous handsets are not compatible. Thanks for suggestions. 

Companies are just not designing and marketing new products for this space.  The market seems to be dying off.  Yes, I still have home analog phones as well, but we are not a growing population.