I recently acquired a new-in-box DiamondTel Model 92 portable cellular phone. This is the exact same model of phone that was used for the official telephone accessory in 92-93 3000GT/Stealth in the US. Here’s a video of the satisfying moment when I peel the protective plastic film off the display (also, I power up the phone and dial a number for the nostalgic dialing tones).
More info about the phone system can be found in this 3SWiki article: Accessory: Cellular Telephone - 3000GT/Stealth Wiki
Also a reminder that I am still looking for a couple of the parts that were used to integrate this phone with the 3000GT, so please contact me if you have any of the related parts and would be willing to sell.