I do, and he wasn't sure. But he was suggesting and attempting to convey the same concept that you're stating it seems.
That is: Each "expandable" Handset (like for a traditional non-SIP Phone handset) would count as a "seat" (he didn't use that term) -- and therefore would cost for each handset. And Probably because the Main # (Customer-facing # -- or -- our old #) has to talk to a "Direct #" (which is a different #) for each handset -- and each # costs.
i.e. -- for the below Amazon Panasonic phone -- we would have to pay for 5 "seats" monthly (and that wasn't ideal at the moment):