The most notable upgrade is the display on top: the old one was just a gimmicky name badge glued to the front and displaying the same scrolling text all the time, but this one is a MAX7219 32x8 LED display that's controlled by the Pi, so it is reactive to what you type. Challenges included: finding any kind of enclosure that it would reasonably fit inside; getting a friend with a machine shop to cut said enclosure down to size; and drilling holes into the top of the phone to attach it and route the cables. Seriously, that took hours. Payphones are made of stern stuff, and I don't have a machine shop.
Instead of using that nightmarish Cirrus audio card that I used last time, I just got a tiny $8 USB audio interface with 3.5mm TRRS. That all went much more sanely.
Also I assembled a little 5v 6w audio amp, so the handset is now loud, possibly even loud enough to be heard in a nightclub.
That necessitated replacing the speaker in the handset, since the old one was clipping with the new amp turned all the way up. And that necessitated getting a new handset, since I couldn't unscrew the endcaps from the old one, even with a giant pipe wrench. Either they were glued on, or they had fused after who-knows-how-many decades melting in the Sun.
And since the amp is stereo, I used the spare channel to attach a second speaker to the inside of the phone case. I never worked out how to drive the solenoids on the physical bell clapper, but now I can play an MP3 that sounds kind of close, and so I have an "attract mode": every now and then the phone rings and the display says something to the effect of "IT'S FOR YOU".