Cisco 7940 SIP With Asterisk FreePBX Features

I have 2 ciscos 7940 already with sip firmware but they are not registering with the Asterisk box. I need them to register, but i need them to work with asterisk features. (forwarding, conference, intercom, voicemail. basically, the more the better) Someone mentioned to me to put sccp on the 7940s and then add something to asterisk for them to work and that was supposed to be better. I just need the 7940s to be reliable with the asterisk and have the features. I need someone that have worked with 7940s and asterisk specifically.