have you configured your ISDN with the following types of information:
ISDN signaling?
Channel Speed? (56k or 64k)
(these paramters should be supplied by your provider, (LEC))
also, for the 3522, it is stated in the documentation to request from the LEC (telco) to turn off the following settings:
packet mode data on the D channel (voice only)
terminal display text
EKTS
call appearances
key hold
ACO
please see the following link for more 3500 GW Configuration:
http://www.cisco.com/en/US/products/hw/video/ps1870/products_administration_guide_book09186a00806f9ff5.html