asterisk users - Oct 2004 - Asterisk- Cisco 7912G second call problem

Iam trying to configure a cisco 7912G (SIP) to work with asterisk. 
Everyting works but when the phone is in use and a second call arrives I 
get Got SIP response 488 "Not Acceptable Here"  and the call is 
transfered to voicemail. Call waiting is already activated in the phone. 
Does somebody is using 7912G with asterisk and is able to receive 2 calls?