^^ He's right. I remember over 10 years ago, AT&T at first gently persuaded, then strongly cajoled, and finally cut off it's customers with 90's era cell phones. They were instituting new tech that was not compatible with their older stuff. At first AT&T offered discounts, then free replacements, then you got cut off. By 2005 or so when the final switch occurred, only maybe 1-3% of their customers had these old tech phones anyway. At the time, not only did I hear it on the news, but my uncle was pained and a bit upset as his 1996 vintage cell was going to become a brick- he liked his phone and didn't want to switch- but he did.
If the geeks are up to it (I'm not one, just proposing here) there could be two ways to go with this. One would be to adapt a Bluetooth transceiver in some way to cars existing mic/audio equipment. That way you'd get factory functionality for your smartphone.
The other way would be to make the handset functional by wiring in the guts of a cheap cellphone. Some things that come to mind are power compatibility, wiring in the audio I/O, and getting the keyboard and display connected. To a tech or an EE figuring the design out might not be so hard. Soldering and screwing it all together could be. Anyone out there up to it?
One could check with the shops in Hemmings that restore/update collectible car radios. IIRC, there are some that, for example, can take a '57 Bel Air stock AM Delco, gut it and give it BT or aux input and stereo output. New guts, but totally stock in the dash- yet compatible with smartphones.
One things for sure- Don't toss it! Folks wii pay a few hundred dollars for a tissue dispenser they'll never use to stick in the '64 impala SS. When these rigs get older there will be authenticity freaks who wii crave that old factory cell.