The Spex uses a MEID code instead of an ESN. The MEID format is newer than ESN--there are more digits in each MEID, too.
If you have an ESN-based phone on your account now, you might be able to get your 6700 to work transparently, but
AFAIK it's not possible to emulate a MEID phone with an ESN one.
<http://www.howardforums.com/showpost.php?p=11902065&postcount=4>
Good luck.