The dlp 73510 is labeled as expandable on the At&t site. It is not.

OK. Page 141 of the manual agrees with you. No cordless system has an infinite number of handsets you can add. 

My post was a statement for people who didn't already know. The set is advertised as expandable. It isn't. That information is not readily available prior to purchasing the unit. The unit is clearly labeled as expandable when it comes with the maximum number of handsets that it can accept. How many handsets a base can accept and which handsets it is compatible with should be listed in the specs for each unit so that potential buyers have access to this information before they purchase a unit. 

If the expansion handsets that fit a certain model are no longer available or no longer being sold by any retailers, then that information should be made readily available prior to the purchase as well.

I and the manual agreed with your original statement.