SIM works in any GSM phone. The problem arises if the phone is locked t
a different service provider than the one you are using.  In that cas
you have to get it unlocked--see other forums for more info poste
about unlocking.
Another issue is that the replacement phone works on the same frequenc
your provider operates on --(850,900,1800,1900), but that has nothing t
do with whether the SIM will work. And the issue regarding freq's can b
searched here as well

Yes, you can use it in your 7100, but will not be able to use data on it
until you call in to Nextel Care (800-639-6111) and add a data plan and
update your equipment on the account.