Can you be little more specific,
for what I understand I will advise you to verify the configuration of your E1.
1-Be sure that you have correctly setup the end point configuration support all your NNX.
2-If there is no restriction in the Cisco and you still get a busy tone verify the configuration and limitation of your E1 at the CO Level.
But by experience I will suggest you to pay an extra attention to the configuration in the Cisco.
Depending what type of Gatekeeper you use it may send you the answer between deny or busy.
Good luck
you can reach me at jkepaou@sun.net.bb if you need more help