Nextel actually does use sim cards in their phones. Here is a snippet from the 190c user manual.
Your phone comes with a built-in level of security protection through the use of the SIM (Subscriber Identity Module) card. The SIM card stores all your Phonebook information and your Nextel account identification information.
Since this information is stored on the SIM card, not in your phone itself, youcan remove the information by removing the SIM card.
NOTE: Except for making emergency calls, your phone will not
function without the SIM card.
The SIM card is located in the SIM card holder on the back of the phone, underneath the battery. You can verify the SIM card is in place by removing the battery and viewing the SIM card through the SIM card cover.