How to establish video chat between two clients?

as i have already wrotten a IAX client capable of establishing two clients need to establish voice chat ,i supposed that ,after merely adding one new video encoder to the client ,the client will support video chat (herein i just let the protocol and network work first disregarding the implement of playing video using such as DirectX ).but in fact after my adding H261 support to the client and sending NEW command with the format of H261 to the asterisk ,it gave the prompt of “Don’t know any of 0x40000 formats,…requested/capability 0x40000/0x40000 incompatible with our capability 0x1fffff.”.
if possible , please recommend or give me some related information about how to write the dial-plan and the implement of the protocol about how to establish video chat .
Thank you!!