The Nextel network is different from all the other cellular networks.. In fact, Nextel isn't "Cellular", it's SMR (specialized mobile radio), with telephone interconnect.
The Nextel network uses a signalling format called IDEN, which is used by a very few companies (Nextel, Southern LINC, Clear-Net, and Boost), the last two I had never heard of until I checked google..
Everybody else uses CDMA, or TDMA (GSM), or legacy AMPS.
The frequency plan is different as well.. Nextel is in the SMR/public safety portion of the 800mhz band (806-821/851-866), while cellular is in the cellular portion of 800 (824-849/869-894), as well as 1900 (1850-1910/1930-1990)..
while it is common to have multi-band/multi-mode phones, I've never seen one offered with IDEN as one of the modes, although I have not looked recently.. this may be changing with the merger of Sprint and Nextel (sprint uses 1900 CDMA)
Nextel will be undergoing some major changes in the next few years, as they migrate from 800 to 1900, and away from IDEN technology, and towards 3G/4G.
https://www.privateline.com/PCS/Frequencies.htm https://electronics.howstuffworks.com/question530.htm https://www.dailywireless.org/modules.php?name=News&file=article&sid=2788
