Is this a production phone or a Frankenphone ?
The handset caps are squared off, and obvious modular cord.
D/P
That's a type K handset, the next evolutionary step after the G series.
They're commonly used on modern office telephones and the like.
They don't really fit very well on a 500/2500, although they will work. Thus, I would guess that someone's probably put that on there, rather than it being a factory installation.
I've seen those a few times on off-brand 2500s, but never on a 500, and never on a Bell System phone. Besides business use as Ben mentioned, they're used on a lot of the Design Line phones like Exeter and Telstar.
Avaya, which is what was once Lucent and before that AT&T Technologies and before that Western Electric, still makes a 2500 style phone which they have updated to have a K series handset.
It actually doesn't look bad on the 2500. It fits the TT's boxier case a bit better than the G, I think.
I have those kind of handsets on my AT&T phones
Getting the caps off, has stratches written all over it. I picture a small screw driver distorting the handset around thye little slot to remove the cap.
I like round.
D/P