Here's a weird one, the Motorola i2000, the first (and one of the only) iDEN phones with a GSM radio
This is an interesting phone. It was the first iDEN phone Nextel sold that had a built in GSM (GSM 900 only) radio for international roaming. Before this phone, the only solution world-traveling Nextel customers had if they wanted to use their phone number abroad was to swap their sim into a GSM handset. Nextel sold phones for this very purpose such as the Motorola V60g, T720, and Timeport 280.
I assume this concept of a GSM roaming iDEN phone never really caught on because Motorola only released a couple other phones with this feature. This phone received an update, the i2000 Plus which added a wap browser. The only other GSM capable iDEN handsets were released in 2005, the Motorola i920/i930 which had tri-band (900/1800/1900) GSM radios. From 2005 until the end of iDEN in the early to mid 2010s, no other GSM capable iDEN handsets were ever released.
For those of you as into these old iDEN phones as I am, the i2000/i2000 Plus are odd. Unlike the other iDEN handsets from this era (such as the i1000), the i2000 needs a SIM to function. The included sim card is used for both iDEN and GSM functionality. The other phones from this era had no sim card, much like the CDMA and D-AMPS phones of the time.
For those of you wondering, I'm using this phone on my private Osmocom network. If it wasn't for this, I wouldn't be able to use this phone since this phone is locked to Nextel and I live in the US where GSM 900 never existed.