I am using asterisk 1.2.13 and the IP phone is Polycom IP 500. I have tried add dtmfmode=rfc2833 into sip.conf, and also in peer configuration, the result is same. So, I think 2b does not support rfc2833. The problem is when I am using dtmfmode=inband to connect between asterisk and 2b, If I dial the digits quite fast, the far end side receive not correct digit. I am sure asterisk has received the digits properly, but seems that asterisk did not sent it out the digits one by one in inband mode. For example, the mobile phones (GSM or 3g) will relay the dtmf digits and then send it out one by one, so i think asterisk is not doing that.