Unfortunately, we are not familiarized with Cisco IP 6921 Family phones since this isn't a product develop by Digium. I suggest to read the manual or administration guide of this phone, so you could determine which settings you need to change.
For more information about how to create a SIP peer in Asterisk, please read the sip.conf on your Asterisk's Directory or visit the following link
All
Unfortunately, we are not familiarized with Cisco IP 6921 Family phones since this isn't a product develop by Digium. I suggest to read the manual or administration guide of this phone, so you could determine which settings you need to change.
For more information about how to create a SIP peer in Asterisk, please read the sip.conf on your Asterisk's Directory or visit the following link
http://svn.digium.com/svn/asterisk/trunk/configs/sip.conf.sample
In this file, you should be able to see some examples of how to create sip peers for various types of phones, including Cisco.
Regards
~Denis Martinez