It might be a pipe somewhere that sets the max bandwidth for calls. Are you calling SIP to H323 or SIP to SIP?
What CallControl are you using? CUCM, VCS or something else?
Have you verified, that the region that covers that device in the CUCM has not video bandwidth set as default? This limits any video call and therefore one should increas that setting to max.
Given that the maximum call rate for an SX10 is 3072, whereas other endpoints can be higher (e.g. 6000/10000) is it possible you've configured the device in CUCM to have a call rate of something higher than 3072, then the SX10 has ignored this value and defaulted back to 384kbps? If you web browse to the codec, what's the maximum bandwidth and maximum total call bandwidth?
If all else fails, have you tried upgrading to the latest firmware?